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riscv: VMAP_STACK overflow detection thread-safe

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commit 31da94c25a ("riscv: add VMAP_STACK overflow detection") added
support for CONFIG_VMAP_STACK. If overflow is detected, CPU switches to
`shadow_stack` temporarily before switching finally to per-cpu
`overflow_stack`.

If two CPUs/harts are racing and end up in over flowing kernel stack, one
or both will end up corrupting each other state because `shadow_stack` is
not per-cpu. This patch optimizes per-cpu overflow stack switch by
directly picking per-cpu `overflow_stack` and gets rid of `shadow_stack`.

Following are the changes in this patch

 - Defines an asm macro to obtain per-cpu symbols in destination
   register.
 - In entry.S, when overflow is detected, per-cpu overflow stack is
   located using per-cpu asm macro. Computing per-cpu symbol requires
   a temporary register. x31 is saved away into CSR_SCRATCH
   (CSR_SCRATCH is anyways zero since we're in kernel).

Please see Links for additional relevant disccussion and alternative
solution.

Tested by `echo EXHAUST_STACK > /sys/kernel/debug/provoke-crash/DIRECT`
Kernel crash log below

 Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT
 Task stack:     [0xff20000010a98000..0xff20000010a9c000]
 Overflow stack: [0xff600001f7d98370..0xff600001f7d99370]
 CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
 Hardware name: riscv-virtio,qemu (DT)
 epc : __memset+0x60/0xfc
  ra : recursive_loop+0x48/0xc6 [lkdtm]
 epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80
  gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88
  t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0
  s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000
  a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000
  a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff
  s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90
  s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684
  s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10
  s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4
  t5 : ffffffff815dbab8 t6 : ff20000010a9bb48
 status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f
 Kernel panic - not syncing: Kernel stack overflow
 CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
 Hardware name: riscv-virtio,qemu (DT)
 Call Trace:
 [<ffffffff80006754>] dump_backtrace+0x30/0x38
 [<ffffffff808de798>] show_stack+0x40/0x4c
 [<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c
 [<ffffffff808ea2d8>] dump_stack+0x18/0x20
 [<ffffffff808dec06>] panic+0x126/0x2fe
 [<ffffffff800065ea>] walk_stackframe+0x0/0xf0
 [<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm]
 SMP: stopping secondary CPUs
 ---[ end Kernel panic - not syncing: Kernel stack overflow ]---

Cc: Guo Ren <guoren@kernel.org>
Cc: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t
Link: https://lore.kernel.org/lkml/20221124094845.1907443-1-debug@rivosinc.com/
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Co-developed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Guo Ren <guoren@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20230927224757.1154247-9-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Deepak Gupta 2023-09-27 22:47:59 +00:00 committed by Palmer Dabbelt
parent 0bb80ecc33
commit be97d0db5f
No known key found for this signature in database
GPG Key ID: 2E1319F35FBB1889
6 changed files with 34 additions and 99 deletions
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1
arch/riscv/include/asm/asm-prototypes.h
View File

@ -25,7 +25,6 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_s);
DECLARE_DO_ERROR_INFO(do_trap_ecall_m);
DECLARE_DO_ERROR_INFO(do_trap_break);
asmlinkage unsigned long get_overflow_stack(void);
asmlinkage void handle_bad_stack(struct pt_regs *regs);
asmlinkage void do_page_fault(struct pt_regs *regs);
asmlinkage void do_irq(struct pt_regs *regs);

22
arch/riscv/include/asm/asm.h
View File

@ -82,6 +82,28 @@
.endr
.endm
#ifdef CONFIG_SMP
#ifdef CONFIG_32BIT
#define PER_CPU_OFFSET_SHIFT 2
#else
#define PER_CPU_OFFSET_SHIFT 3
#endif
.macro asm_per_cpu dst sym tmp
REG_L \tmp, TASK_TI_CPU_NUM(tp)
slli \tmp, \tmp, PER_CPU_OFFSET_SHIFT
la \dst, __per_cpu_offset
add \dst, \dst, \tmp
REG_L \tmp, 0(\dst)
la \dst, \sym
add \dst, \dst, \tmp
.endm
#else /* CONFIG_SMP */
.macro asm_per_cpu dst sym tmp
la \dst, \sym
.endm
#endif /* CONFIG_SMP */
/* save all GPs except x1 ~ x5 */
.macro save_from_x6_to_x31
REG_S x6, PT_T1(sp)

3
arch/riscv/include/asm/thread_info.h
View File

@ -34,9 +34,6 @@
#ifndef __ASSEMBLY__
extern long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE / sizeof(long)];
extern unsigned long spin_shadow_stack;
#include <asm/processor.h>
#include <asm/csr.h>

1
arch/riscv/kernel/asm-offsets.c
View File

@ -39,6 +39,7 @@ void asm_offsets(void)
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]);
OFFSET(TASK_THREAD_F2, task_struct, thread.fstate.f[2]);

70
arch/riscv/kernel/entry.S
View File

@ -10,9 +10,11 @@
#include <asm/asm.h>
#include <asm/csr.h>
#include <asm/unistd.h>
#include <asm/page.h>
#include <asm/thread_info.h>
#include <asm/asm-offsets.h>
#include <asm/errata_list.h>
#include <linux/sizes.h>
SYM_CODE_START(handle_exception)
/*
@ -170,67 +172,15 @@ SYM_CODE_END(ret_from_exception)
#ifdef CONFIG_VMAP_STACK
SYM_CODE_START_LOCAL(handle_kernel_stack_overflow)
/*
* Takes the psuedo-spinlock for the shadow stack, in case multiple
* harts are concurrently overflowing their kernel stacks. We could
* store any value here, but since we're overflowing the kernel stack
* already we only have SP to use as a scratch register. So we just
* swap in the address of the spinlock, as that's definately non-zero.
*
* Pairs with a store_release in handle_bad_stack().
*/
1: la sp, spin_shadow_stack
REG_AMOSWAP_AQ sp, sp, (sp)
bnez sp, 1b
/* we reach here from kernel context, sscratch must be 0 */
csrrw x31, CSR_SCRATCH, x31
asm_per_cpu sp, overflow_stack, x31
li x31, OVERFLOW_STACK_SIZE
add sp, sp, x31
/* zero out x31 again and restore x31 */
xor x31, x31, x31
csrrw x31, CSR_SCRATCH, x31
la sp, shadow_stack
addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
//save caller register to shadow stack
addi sp, sp, -(PT_SIZE_ON_STACK)
REG_S x1, PT_RA(sp)
REG_S x5, PT_T0(sp)
REG_S x6, PT_T1(sp)
REG_S x7, PT_T2(sp)
REG_S x10, PT_A0(sp)
REG_S x11, PT_A1(sp)
REG_S x12, PT_A2(sp)
REG_S x13, PT_A3(sp)
REG_S x14, PT_A4(sp)
REG_S x15, PT_A5(sp)
REG_S x16, PT_A6(sp)
REG_S x17, PT_A7(sp)
REG_S x28, PT_T3(sp)
REG_S x29, PT_T4(sp)
REG_S x30, PT_T5(sp)
REG_S x31, PT_T6(sp)
la ra, restore_caller_reg
tail get_overflow_stack
restore_caller_reg:
//save per-cpu overflow stack
REG_S a0, -8(sp)
//restore caller register from shadow_stack
REG_L x1, PT_RA(sp)
REG_L x5, PT_T0(sp)
REG_L x6, PT_T1(sp)
REG_L x7, PT_T2(sp)
REG_L x10, PT_A0(sp)
REG_L x11, PT_A1(sp)
REG_L x12, PT_A2(sp)
REG_L x13, PT_A3(sp)
REG_L x14, PT_A4(sp)
REG_L x15, PT_A5(sp)
REG_L x16, PT_A6(sp)
REG_L x17, PT_A7(sp)
REG_L x28, PT_T3(sp)
REG_L x29, PT_T4(sp)
REG_L x30, PT_T5(sp)
REG_L x31, PT_T6(sp)
//load per-cpu overflow stack
REG_L sp, -8(sp)
addi sp, sp, -(PT_SIZE_ON_STACK)
//save context to overflow stack

36
arch/riscv/kernel/traps.c
View File

@ -402,48 +402,14 @@ int is_valid_bugaddr(unsigned long pc)
#endif /* CONFIG_GENERIC_BUG */
#ifdef CONFIG_VMAP_STACK
/*
* Extra stack space that allows us to provide panic messages when the kernel
* has overflowed its stack.
*/
static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
overflow_stack)__aligned(16);
/*
* A temporary stack for use by handle_kernel_stack_overflow. This is used so
* we can call into C code to get the per-hart overflow stack. Usage of this
* stack must be protected by spin_shadow_stack.
*/
long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
/*
* A pseudo spinlock to protect the shadow stack from being used by multiple
* harts concurrently. This isn't a real spinlock because the lock side must
* be taken without a valid stack and only a single register, it's only taken
* while in the process of panicing anyway so the performance and error
* checking a proper spinlock gives us doesn't matter.
*/
unsigned long spin_shadow_stack;
asmlinkage unsigned long get_overflow_stack(void)
{
return (unsigned long)this_cpu_ptr(overflow_stack) +
OVERFLOW_STACK_SIZE;
}
asmlinkage void handle_bad_stack(struct pt_regs *regs)
{
unsigned long tsk_stk = (unsigned long)current->stack;
unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);
/*
* We're done with the shadow stack by this point, as we're on the
* overflow stack. Tell any other concurrent overflowing harts that
* they can proceed with panicing by releasing the pseudo-spinlock.
*
* This pairs with an amoswap.aq in handle_kernel_stack_overflow.
*/
smp_store_release(&spin_shadow_stack, 0);
console_verbose();
pr_emerg("Insufficient stack space to handle exception!\n");