ARM: stack protector: change the canary value per task

A new random value for the canary is stored in the task struct whenever
a new task is forked.  This is meant to allow for different canary values
per task.  On ARM, GCC expects the canary value to be found in a global
variable called __stack_chk_guard.  So this variable has to be updated
with the value stored in the task struct whenever a task switch occurs.

Because the variable GCC expects is global, this cannot work on SMP
unfortunately.  So, on SMP, the same initial canary value is kept
throughout, making this feature a bit less effective although it is still
useful.

One way to overcome this GCC limitation would be to locate the
__stack_chk_guard variable into a memory page of its own for each CPU,
and then use TLB locking to have each CPU see its own page at the same
virtual address for each of them.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>

2 files changed, 11 insertions, 0 deletions

diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
index 883511522fca..85f2a019f77b 100644
--- a/arch/arm/kernel/asm-offsets.c
+++ b/arch/arm/kernel/asm-offsets.c
@@ -40,6 +40,9 @@
 int main(void)
 {
   DEFINE(TSK_ACTIVE_MM,         offsetof(struct task_struct, active_mm));
+#ifdef CONFIG_CC_STACKPROTECTOR
+  DEFINE(TSK_STACK_CANARY,      offsetof(struct task_struct, stack_canary));
+#endif
   BLANK();
   DEFINE(TI_FLAGS,              offsetof(struct thread_info, flags));
   DEFINE(TI_PREEMPT,            offsetof(struct thread_info, preempt_count));
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 7ee48e7f8f31..2d14081b26b1 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -745,6 +745,11 @@ ENTRY(__switch_to)
         mov     r4, #0xffff0fff
         str     r3, [r4, #-15]                  @ TLS val at 0xffff0ff0
 #endif
+#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
+        ldr     r7, [r2, #TI_TASK]
+        ldr     r8, =__stack_chk_guard
+        ldr     r7, [r7, #TSK_STACK_CANARY]
+#endif
 #ifdef CONFIG_MMU
         mcr     p15, 0, r6, c3, c0, 0           @ Set domain register
 #endif
@@ -753,6 +758,9 @@ ENTRY(__switch_to)
         ldr     r0, =thread_notify_head
         mov     r1, #THREAD_NOTIFY_SWITCH
         bl      atomic_notifier_call_chain
+#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
+        str     r7, [r8]
+#endif
  THUMB( mov     ip, r4                     )
         mov     r0, r5
  ARM(   ldmia   r4, {r4 - sl, fp, sp, pc}  )    @ Load all regs saved previously