linux/arch/x86/include/asm/mpx.h

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#ifndef _ASM_X86_MPX_H
#define _ASM_X86_MPX_H
#include <linux/types.h>
#include <asm/ptrace.h>
x86, mpx: Decode MPX instruction to get bound violation information This patch sets bound violation fields of siginfo struct in #BR exception handler by decoding the user instruction and constructing the faulting pointer. We have to be very careful when decoding these instructions. They are completely controlled by userspace and may be changed at any time up to and including the point where we try to copy them in to the kernel. They may or may not be MPX instructions and could be completely invalid for all we know. Note: This code is based on Qiaowei Ren's specialized MPX decoder, but uses the generic decoder whenever possible. It was tested for robustness by generating a completely random data stream and trying to decode that stream. I also unmapped random pages inside the stream to test the "partial instruction" short read code. We kzalloc() the siginfo instead of stack allocating it because we need to memset() it anyway, and doing this makes it much more clear when it got initialized by the MPX instruction decoder. Changes from the old decoder: * Use the generic decoder instead of custom functions. Saved ~70 lines of code overall. * Remove insn->addr_bytes code (never used??) * Make sure never to possibly overflow the regoff[] array, plus check the register range correctly in 32 and 64-bit modes. * Allow get_reg() to return an error and have mpx_get_addr_ref() handle when it sees errors. * Only call insn_get_*() near where we actually use the values instead if trying to call them all at once. * Handle short reads from copy_from_user() and check the actual number of read bytes against what we expect from insn_get_length(). If a read stops in the middle of an instruction, we error out. * Actually check the opcodes intead of ignoring them. * Dynamically kzalloc() siginfo_t so we don't leak any stack data. * Detect and handle decoder failures instead of ignoring them. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151828.5BDD0915@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:28 +00:00
#include <asm/insn.h>
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
/*
* NULL is theoretically a valid place to put the bounds
* directory, so point this at an invalid address.
*/
#define MPX_INVALID_BOUNDS_DIR ((void __user *)-1)
#define MPX_BNDCFG_ENABLE_FLAG 0x1
#define MPX_BD_ENTRY_VALID_FLAG 0x1
/*
* The upper 28 bits [47:20] of the virtual address in 64-bit
* are used to index into bounds directory (BD).
*
* The directory is 2G (2^31) in size, and with 8-byte entries
* it has 2^28 entries.
*/
#define MPX_BD_SIZE_BYTES_64 (1UL<<31)
#define MPX_BD_ENTRY_BYTES_64 8
#define MPX_BD_NR_ENTRIES_64 (MPX_BD_SIZE_BYTES_64/MPX_BD_ENTRY_BYTES_64)
/*
* The 32-bit directory is 4MB (2^22) in size, and with 4-byte
* entries it has 2^20 entries.
*/
#define MPX_BD_SIZE_BYTES_32 (1UL<<22)
#define MPX_BD_ENTRY_BYTES_32 4
#define MPX_BD_NR_ENTRIES_32 (MPX_BD_SIZE_BYTES_32/MPX_BD_ENTRY_BYTES_32)
/*
* A 64-bit table is 4MB total in size, and an entry is
* 4 64-bit pointers in size.
*/
#define MPX_BT_SIZE_BYTES_64 (1UL<<22)
#define MPX_BT_ENTRY_BYTES_64 32
#define MPX_BT_NR_ENTRIES_64 (MPX_BT_SIZE_BYTES_64/MPX_BT_ENTRY_BYTES_64)
/*
* A 32-bit table is 16kB total in size, and an entry is
* 4 32-bit pointers in size.
*/
#define MPX_BT_SIZE_BYTES_32 (1UL<<14)
#define MPX_BT_ENTRY_BYTES_32 16
#define MPX_BT_NR_ENTRIES_32 (MPX_BT_SIZE_BYTES_32/MPX_BT_ENTRY_BYTES_32)
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
#define MPX_BNDSTA_TAIL 2
#define MPX_BNDCFG_TAIL 12
#define MPX_BNDSTA_ADDR_MASK (~((1UL<<MPX_BNDSTA_TAIL)-1))
#define MPX_BNDCFG_ADDR_MASK (~((1UL<<MPX_BNDCFG_TAIL)-1))
#define MPX_BNDSTA_ERROR_CODE 0x3
x86, mpx: Decode MPX instruction to get bound violation information This patch sets bound violation fields of siginfo struct in #BR exception handler by decoding the user instruction and constructing the faulting pointer. We have to be very careful when decoding these instructions. They are completely controlled by userspace and may be changed at any time up to and including the point where we try to copy them in to the kernel. They may or may not be MPX instructions and could be completely invalid for all we know. Note: This code is based on Qiaowei Ren's specialized MPX decoder, but uses the generic decoder whenever possible. It was tested for robustness by generating a completely random data stream and trying to decode that stream. I also unmapped random pages inside the stream to test the "partial instruction" short read code. We kzalloc() the siginfo instead of stack allocating it because we need to memset() it anyway, and doing this makes it much more clear when it got initialized by the MPX instruction decoder. Changes from the old decoder: * Use the generic decoder instead of custom functions. Saved ~70 lines of code overall. * Remove insn->addr_bytes code (never used??) * Make sure never to possibly overflow the regoff[] array, plus check the register range correctly in 32 and 64-bit modes. * Allow get_reg() to return an error and have mpx_get_addr_ref() handle when it sees errors. * Only call insn_get_*() near where we actually use the values instead if trying to call them all at once. * Handle short reads from copy_from_user() and check the actual number of read bytes against what we expect from insn_get_length(). If a read stops in the middle of an instruction, we error out. * Actually check the opcodes intead of ignoring them. * Dynamically kzalloc() siginfo_t so we don't leak any stack data. * Detect and handle decoder failures instead of ignoring them. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151828.5BDD0915@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:28 +00:00
#ifdef CONFIG_X86_INTEL_MPX
siginfo_t *mpx_generate_siginfo(struct pt_regs *regs);
int mpx_handle_bd_fault(void);
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
static inline int kernel_managing_mpx_tables(struct mm_struct *mm)
{
return (mm->context.bd_addr != MPX_INVALID_BOUNDS_DIR);
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
}
static inline void mpx_mm_init(struct mm_struct *mm)
{
/*
* NULL is theoretically a valid place to put the bounds
* directory, so point this at an invalid address.
*/
mm->context.bd_addr = MPX_INVALID_BOUNDS_DIR;
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
}
x86, mpx: Cleanup unused bound tables The previous patch allocates bounds tables on-demand. As noted in an earlier description, these can add up to *HUGE* amounts of memory. This has caused OOMs in practice when running tests. This patch adds support for freeing bounds tables when they are no longer in use. There are two types of mappings in play when unmapping tables: 1. The mapping with the actual data, which userspace is munmap()ing or brk()ing away, etc... 2. The mapping for the bounds table *backing* the data (is tagged with VM_MPX, see the patch "add MPX specific mmap interface"). If userspace use the prctl() indroduced earlier in this patchset to enable the management of bounds tables in kernel, when it unmaps the first type of mapping with the actual data, the kernel needs to free the mapping for the bounds table backing the data. This patch hooks in at the very end of do_unmap() to do so. We look at the addresses being unmapped and find the bounds directory entries and tables which cover those addresses. If an entire table is unused, we clear associated directory entry and free the table. Once we unmap the bounds table, we would have a bounds directory entry pointing at empty address space. That address space might now be allocated for some other (random) use, and the MPX hardware might now try to walk it as if it were a bounds table. That would be bad. So any unmapping of an enture bounds table has to be accompanied by a corresponding write to the bounds directory entry to invalidate it. That write to the bounds directory can fault, which causes the following problem: Since we are doing the freeing from munmap() (and other paths like it), we hold mmap_sem for write. If we fault, the page fault handler will attempt to acquire mmap_sem for read and we will deadlock. To avoid the deadlock, we pagefault_disable() when touching the bounds directory entry and use a get_user_pages() to resolve the fault. The unmapping of bounds tables happends under vm_munmap(). We also (indirectly) call vm_munmap() to _do_ the unmapping of the bounds tables. We avoid unbounded recursion by disallowing freeing of bounds tables *for* bounds tables. This would not occur normally, so should not have any practical impact. Being strict about it here helps ensure that we do not have an exploitable stack overflow. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151831.E4531C4A@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:31 +00:00
void mpx_notify_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long start, unsigned long end);
x86, mpx: Decode MPX instruction to get bound violation information This patch sets bound violation fields of siginfo struct in #BR exception handler by decoding the user instruction and constructing the faulting pointer. We have to be very careful when decoding these instructions. They are completely controlled by userspace and may be changed at any time up to and including the point where we try to copy them in to the kernel. They may or may not be MPX instructions and could be completely invalid for all we know. Note: This code is based on Qiaowei Ren's specialized MPX decoder, but uses the generic decoder whenever possible. It was tested for robustness by generating a completely random data stream and trying to decode that stream. I also unmapped random pages inside the stream to test the "partial instruction" short read code. We kzalloc() the siginfo instead of stack allocating it because we need to memset() it anyway, and doing this makes it much more clear when it got initialized by the MPX instruction decoder. Changes from the old decoder: * Use the generic decoder instead of custom functions. Saved ~70 lines of code overall. * Remove insn->addr_bytes code (never used??) * Make sure never to possibly overflow the regoff[] array, plus check the register range correctly in 32 and 64-bit modes. * Allow get_reg() to return an error and have mpx_get_addr_ref() handle when it sees errors. * Only call insn_get_*() near where we actually use the values instead if trying to call them all at once. * Handle short reads from copy_from_user() and check the actual number of read bytes against what we expect from insn_get_length(). If a read stops in the middle of an instruction, we error out. * Actually check the opcodes intead of ignoring them. * Dynamically kzalloc() siginfo_t so we don't leak any stack data. * Detect and handle decoder failures instead of ignoring them. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151828.5BDD0915@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:28 +00:00
#else
static inline siginfo_t *mpx_generate_siginfo(struct pt_regs *regs)
x86, mpx: Decode MPX instruction to get bound violation information This patch sets bound violation fields of siginfo struct in #BR exception handler by decoding the user instruction and constructing the faulting pointer. We have to be very careful when decoding these instructions. They are completely controlled by userspace and may be changed at any time up to and including the point where we try to copy them in to the kernel. They may or may not be MPX instructions and could be completely invalid for all we know. Note: This code is based on Qiaowei Ren's specialized MPX decoder, but uses the generic decoder whenever possible. It was tested for robustness by generating a completely random data stream and trying to decode that stream. I also unmapped random pages inside the stream to test the "partial instruction" short read code. We kzalloc() the siginfo instead of stack allocating it because we need to memset() it anyway, and doing this makes it much more clear when it got initialized by the MPX instruction decoder. Changes from the old decoder: * Use the generic decoder instead of custom functions. Saved ~70 lines of code overall. * Remove insn->addr_bytes code (never used??) * Make sure never to possibly overflow the regoff[] array, plus check the register range correctly in 32 and 64-bit modes. * Allow get_reg() to return an error and have mpx_get_addr_ref() handle when it sees errors. * Only call insn_get_*() near where we actually use the values instead if trying to call them all at once. * Handle short reads from copy_from_user() and check the actual number of read bytes against what we expect from insn_get_length(). If a read stops in the middle of an instruction, we error out. * Actually check the opcodes intead of ignoring them. * Dynamically kzalloc() siginfo_t so we don't leak any stack data. * Detect and handle decoder failures instead of ignoring them. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151828.5BDD0915@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:28 +00:00
{
return NULL;
}
static inline int mpx_handle_bd_fault(void)
x86, mpx: On-demand kernel allocation of bounds tables This is really the meat of the MPX patch set. If there is one patch to review in the entire series, this is the one. There is a new ABI here and this kernel code also interacts with userspace memory in a relatively unusual manner. (small FAQ below). Long Description: This patch adds two prctl() commands to provide enable or disable the management of bounds tables in kernel, including on-demand kernel allocation (See the patch "on-demand kernel allocation of bounds tables") and cleanup (See the patch "cleanup unused bound tables"). Applications do not strictly need the kernel to manage bounds tables and we expect some applications to use MPX without taking advantage of this kernel support. This means the kernel can not simply infer whether an application needs bounds table management from the MPX registers. The prctl() is an explicit signal from userspace. PR_MPX_ENABLE_MANAGEMENT is meant to be a signal from userspace to require kernel's help in managing bounds tables. PR_MPX_DISABLE_MANAGEMENT is the opposite, meaning that userspace don't want kernel's help any more. With PR_MPX_DISABLE_MANAGEMENT, the kernel won't allocate and free bounds tables even if the CPU supports MPX. PR_MPX_ENABLE_MANAGEMENT will fetch the base address of the bounds directory out of a userspace register (bndcfgu) and then cache it into a new field (->bd_addr) in the 'mm_struct'. PR_MPX_DISABLE_MANAGEMENT will set "bd_addr" to an invalid address. Using this scheme, we can use "bd_addr" to determine whether the management of bounds tables in kernel is enabled. Also, the only way to access that bndcfgu register is via an xsaves, which can be expensive. Caching "bd_addr" like this also helps reduce the cost of those xsaves when doing table cleanup at munmap() time. Unfortunately, we can not apply this optimization to #BR fault time because we need an xsave to get the value of BNDSTATUS. ==== Why does the hardware even have these Bounds Tables? ==== MPX only has 4 hardware registers for storing bounds information. If MPX-enabled code needs more than these 4 registers, it needs to spill them somewhere. It has two special instructions for this which allow the bounds to be moved between the bounds registers and some new "bounds tables". They are similar conceptually to a page fault and will be raised by the MPX hardware during both bounds violations or when the tables are not present. This patch handles those #BR exceptions for not-present tables by carving the space out of the normal processes address space (essentially calling the new mmap() interface indroduced earlier in this patch set.) and then pointing the bounds-directory over to it. The tables *need* to be accessed and controlled by userspace because the instructions for moving bounds in and out of them are extremely frequent. They potentially happen every time a register pointing to memory is dereferenced. Any direct kernel involvement (like a syscall) to access the tables would obviously destroy performance. ==== Why not do this in userspace? ==== This patch is obviously doing this allocation in the kernel. However, MPX does not strictly *require* anything in the kernel. It can theoretically be done completely from userspace. Here are a few ways this *could* be done. I don't think any of them are practical in the real-world, but here they are. Q: Can virtual space simply be reserved for the bounds tables so that we never have to allocate them? A: As noted earlier, these tables are *HUGE*. An X-GB virtual area needs 4*X GB of virtual space, plus 2GB for the bounds directory. If we were to preallocate them for the 128TB of user virtual address space, we would need to reserve 512TB+2GB, which is larger than the entire virtual address space today. This means they can not be reserved ahead of time. Also, a single process's pre-popualated bounds directory consumes 2GB of virtual *AND* physical memory. IOW, it's completely infeasible to prepopulate bounds directories. Q: Can we preallocate bounds table space at the same time memory is allocated which might contain pointers that might eventually need bounds tables? A: This would work if we could hook the site of each and every memory allocation syscall. This can be done for small, constrained applications. But, it isn't practical at a larger scale since a given app has no way of controlling how all the parts of the app might allocate memory (think libraries). The kernel is really the only place to intercept these calls. Q: Could a bounds fault be handed to userspace and the tables allocated there in a signal handler instead of in the kernel? A: (thanks to tglx) mmap() is not on the list of safe async handler functions and even if mmap() would work it still requires locking or nasty tricks to keep track of the allocation state there. Having ruled out all of the userspace-only approaches for managing bounds tables that we could think of, we create them on demand in the kernel. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151829.AD4310DE@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:29 +00:00
{
return -EINVAL;
}
static inline int kernel_managing_mpx_tables(struct mm_struct *mm)
{
return 0;
}
static inline void mpx_mm_init(struct mm_struct *mm)
{
}
x86, mpx: Cleanup unused bound tables The previous patch allocates bounds tables on-demand. As noted in an earlier description, these can add up to *HUGE* amounts of memory. This has caused OOMs in practice when running tests. This patch adds support for freeing bounds tables when they are no longer in use. There are two types of mappings in play when unmapping tables: 1. The mapping with the actual data, which userspace is munmap()ing or brk()ing away, etc... 2. The mapping for the bounds table *backing* the data (is tagged with VM_MPX, see the patch "add MPX specific mmap interface"). If userspace use the prctl() indroduced earlier in this patchset to enable the management of bounds tables in kernel, when it unmaps the first type of mapping with the actual data, the kernel needs to free the mapping for the bounds table backing the data. This patch hooks in at the very end of do_unmap() to do so. We look at the addresses being unmapped and find the bounds directory entries and tables which cover those addresses. If an entire table is unused, we clear associated directory entry and free the table. Once we unmap the bounds table, we would have a bounds directory entry pointing at empty address space. That address space might now be allocated for some other (random) use, and the MPX hardware might now try to walk it as if it were a bounds table. That would be bad. So any unmapping of an enture bounds table has to be accompanied by a corresponding write to the bounds directory entry to invalidate it. That write to the bounds directory can fault, which causes the following problem: Since we are doing the freeing from munmap() (and other paths like it), we hold mmap_sem for write. If we fault, the page fault handler will attempt to acquire mmap_sem for read and we will deadlock. To avoid the deadlock, we pagefault_disable() when touching the bounds directory entry and use a get_user_pages() to resolve the fault. The unmapping of bounds tables happends under vm_munmap(). We also (indirectly) call vm_munmap() to _do_ the unmapping of the bounds tables. We avoid unbounded recursion by disallowing freeing of bounds tables *for* bounds tables. This would not occur normally, so should not have any practical impact. Being strict about it here helps ensure that we do not have an exploitable stack overflow. Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151831.E4531C4A@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:31 +00:00
static inline void mpx_notify_unmap(struct mm_struct *mm,
struct vm_area_struct *vma,
unsigned long start, unsigned long end)
{
}
x86, mpx: Decode MPX instruction to get bound violation information This patch sets bound violation fields of siginfo struct in #BR exception handler by decoding the user instruction and constructing the faulting pointer. We have to be very careful when decoding these instructions. They are completely controlled by userspace and may be changed at any time up to and including the point where we try to copy them in to the kernel. They may or may not be MPX instructions and could be completely invalid for all we know. Note: This code is based on Qiaowei Ren's specialized MPX decoder, but uses the generic decoder whenever possible. It was tested for robustness by generating a completely random data stream and trying to decode that stream. I also unmapped random pages inside the stream to test the "partial instruction" short read code. We kzalloc() the siginfo instead of stack allocating it because we need to memset() it anyway, and doing this makes it much more clear when it got initialized by the MPX instruction decoder. Changes from the old decoder: * Use the generic decoder instead of custom functions. Saved ~70 lines of code overall. * Remove insn->addr_bytes code (never used??) * Make sure never to possibly overflow the regoff[] array, plus check the register range correctly in 32 and 64-bit modes. * Allow get_reg() to return an error and have mpx_get_addr_ref() handle when it sees errors. * Only call insn_get_*() near where we actually use the values instead if trying to call them all at once. * Handle short reads from copy_from_user() and check the actual number of read bytes against what we expect from insn_get_length(). If a read stops in the middle of an instruction, we error out. * Actually check the opcodes intead of ignoring them. * Dynamically kzalloc() siginfo_t so we don't leak any stack data. * Detect and handle decoder failures instead of ignoring them. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Based-on-patch-by: Qiaowei Ren <qiaowei.ren@intel.com> Cc: linux-mm@kvack.org Cc: linux-mips@linux-mips.org Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141114151828.5BDD0915@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2014-11-14 15:18:28 +00:00
#endif /* CONFIG_X86_INTEL_MPX */
#endif /* _ASM_X86_MPX_H */