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arm64/gcs: Document the ABI for Guarded Control Stacks
Add some documentation of the userspace ABI for Guarded Control Stacks. Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Acked-by: Yury Khrustalev <yury.khrustalev@arm.com> Signed-off-by: Mark Brown <broonie@kernel.org> Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-7-222b78d87eee@kernel.org Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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Catalin Marinas
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Documentation/arch/arm64/gcs.rst
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Documentation/arch/arm64/gcs.rst
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===============================================
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Guarded Control Stack support for AArch64 Linux
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===============================================
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This document outlines briefly the interface provided to userspace by Linux in
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order to support use of the ARM Guarded Control Stack (GCS) feature.
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This is an outline of the most important features and issues only and not
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intended to be exhaustive.
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1. General
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-----------
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* GCS is an architecture feature intended to provide greater protection
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against return oriented programming (ROP) attacks and to simplify the
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implementation of features that need to collect stack traces such as
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profiling.
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* When GCS is enabled a separate guarded control stack is maintained by the
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PE which is writeable only through specific GCS operations. This
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stores the call stack only, when a procedure call instruction is
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performed the current PC is pushed onto the GCS and on RET the
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address in the LR is verified against that on the top of the GCS.
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* When active the current GCS pointer is stored in the system register
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GCSPR_EL0. This is readable by userspace but can only be updated
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via specific GCS instructions.
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* The architecture provides instructions for switching between guarded
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control stacks with checks to ensure that the new stack is a valid
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target for switching.
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* The functionality of GCS is similar to that provided by the x86 Shadow
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Stack feature, due to sharing of userspace interfaces the ABI refers to
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shadow stacks rather than GCS.
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* Support for GCS is reported to userspace via HWCAP_GCS in the aux vector
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AT_HWCAP2 entry.
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* GCS is enabled per thread. While there is support for disabling GCS
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at runtime this should be done with great care.
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* GCS memory access faults are reported as normal memory access faults.
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* GCS specific errors (those reported with EC 0x2d) will be reported as
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SIGSEGV with a si_code of SEGV_CPERR (control protection error).
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* GCS is supported only for AArch64.
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* On systems where GCS is supported GCSPR_EL0 is always readable by EL0
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regardless of the GCS configuration for the thread.
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* The architecture supports enabling GCS without verifying that return values
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in LR match those in the GCS, the LR will be ignored. This is not supported
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by Linux.
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2. Enabling and disabling Guarded Control Stacks
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-------------------------------------------------
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* GCS is enabled and disabled for a thread via the PR_SET_SHADOW_STACK_STATUS
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prctl(), this takes a single flags argument specifying which GCS features
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should be used.
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* When set PR_SHADOW_STACK_ENABLE flag allocates a Guarded Control Stack
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and enables GCS for the thread, enabling the functionality controlled by
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GCSCRE0_EL1.{nTR, RVCHKEN, PCRSEL}.
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* When set the PR_SHADOW_STACK_PUSH flag enables the functionality controlled
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by GCSCRE0_EL1.PUSHMEn, allowing explicit GCS pushes.
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* When set the PR_SHADOW_STACK_WRITE flag enables the functionality controlled
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by GCSCRE0_EL1.STREn, allowing explicit stores to the Guarded Control Stack.
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* Any unknown flags will cause PR_SET_SHADOW_STACK_STATUS to return -EINVAL.
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* PR_LOCK_SHADOW_STACK_STATUS is passed a bitmask of features with the same
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values as used for PR_SET_SHADOW_STACK_STATUS. Any future changes to the
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status of the specified GCS mode bits will be rejected.
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* PR_LOCK_SHADOW_STACK_STATUS allows any bit to be locked, this allows
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userspace to prevent changes to any future features.
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* There is no support for a process to remove a lock that has been set for
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it.
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* PR_SET_SHADOW_STACK_STATUS and PR_LOCK_SHADOW_STACK_STATUS affect only the
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thread that called them, any other running threads will be unaffected.
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* New threads inherit the GCS configuration of the thread that created them.
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* GCS is disabled on exec().
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* The current GCS configuration for a thread may be read with the
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PR_GET_SHADOW_STACK_STATUS prctl(), this returns the same flags that
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are passed to PR_SET_SHADOW_STACK_STATUS.
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* If GCS is disabled for a thread after having previously been enabled then
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the stack will remain allocated for the lifetime of the thread. At present
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any attempt to reenable GCS for the thread will be rejected, this may be
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revisited in future.
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* It should be noted that since enabling GCS will result in GCS becoming
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active immediately it is not normally possible to return from the function
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that invoked the prctl() that enabled GCS. It is expected that the normal
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usage will be that GCS is enabled very early in execution of a program.
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3. Allocation of Guarded Control Stacks
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----------------------------------------
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* When GCS is enabled for a thread a new Guarded Control Stack will be
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allocated for it of half the standard stack size or 2 gigabytes,
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whichever is smaller.
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* When a new thread is created by a thread which has GCS enabled then a
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new Guarded Control Stack will be allocated for the new thread with
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half the size of the standard stack.
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* When a stack is allocated by enabling GCS or during thread creation then
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the top 8 bytes of the stack will be initialised to 0 and GCSPR_EL0 will
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be set to point to the address of this 0 value, this can be used to
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detect the top of the stack.
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* Additional Guarded Control Stacks can be allocated using the
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map_shadow_stack() system call.
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* Stacks allocated using map_shadow_stack() can optionally have an end of
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stack marker and cap placed at the top of the stack. If the flag
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SHADOW_STACK_SET_TOKEN is specified a cap will be placed on the stack,
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if SHADOW_STACK_SET_MARKER is not specified the cap will be the top 8
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bytes of the stack and if it is specified then the cap will be the next
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8 bytes. While specifying just SHADOW_STACK_SET_MARKER by itself is
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valid since the marker is all bits 0 it has no observable effect.
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* Stacks allocated using map_shadow_stack() must have a size which is a
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multiple of 8 bytes larger than 8 bytes and must be 8 bytes aligned.
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* An address can be specified to map_shadow_stack(), if one is provided then
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it must be aligned to a page boundary.
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* When a thread is freed the Guarded Control Stack initially allocated for
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that thread will be freed. Note carefully that if the stack has been
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switched this may not be the stack currently in use by the thread.
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4. Signal handling
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--------------------
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* A new signal frame record gcs_context encodes the current GCS mode and
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pointer for the interrupted context on signal delivery. This will always
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be present on systems that support GCS.
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* The record contains a flag field which reports the current GCS configuration
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for the interrupted context as PR_GET_SHADOW_STACK_STATUS would.
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* The signal handler is run with the same GCS configuration as the interrupted
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context.
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* When GCS is enabled for the interrupted thread a signal handling specific
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GCS cap token will be written to the GCS, this is an architectural GCS cap
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with the token type (bits 0..11) all clear. The GCSPR_EL0 reported in the
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signal frame will point to this cap token.
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* The signal handler will use the same GCS as the interrupted context.
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* When GCS is enabled on signal entry a frame with the address of the signal
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return handler will be pushed onto the GCS, allowing return from the signal
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handler via RET as normal. This will not be reported in the gcs_context in
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the signal frame.
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5. Signal return
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-----------------
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When returning from a signal handler:
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* If there is a gcs_context record in the signal frame then the GCS flags
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and GCSPR_EL0 will be restored from that context prior to further
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validation.
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* If there is no gcs_context record in the signal frame then the GCS
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configuration will be unchanged.
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* If GCS is enabled on return from a signal handler then GCSPR_EL0 must
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point to a valid GCS signal cap record, this will be popped from the
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GCS prior to signal return.
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* If the GCS configuration is locked when returning from a signal then any
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attempt to change the GCS configuration will be treated as an error. This
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is true even if GCS was not enabled prior to signal entry.
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* GCS may be disabled via signal return but any attempt to enable GCS via
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signal return will be rejected.
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6. ptrace extensions
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---------------------
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* A new regset NT_ARM_GCS is defined for use with PTRACE_GETREGSET and
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PTRACE_SETREGSET.
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* Due to the complexity surrounding allocation and deallocation of stacks and
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lack of practical application it is not possible to enable GCS via ptrace.
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GCS may be disabled via the ptrace interface.
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* Other GCS modes may be configured via ptrace.
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* Configuration via ptrace ignores locking of GCS mode bits.
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7. ELF coredump extensions
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---------------------------
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* NT_ARM_GCS notes will be added to each coredump for each thread of the
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dumped process. The contents will be equivalent to the data that would
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have been read if a PTRACE_GETREGSET of the corresponding type were
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executed for each thread when the coredump was generated.
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8. /proc extensions
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--------------------
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* Guarded Control Stack pages will include "ss" in their VmFlags in
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/proc/<pid>/smaps.
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@ -15,6 +15,7 @@ ARM64 Architecture
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cpu-feature-registers
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cpu-hotplug
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elf_hwcaps
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gcs
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hugetlbpage
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kdump
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legacy_instructions
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