[PATCH 2/2] powerpc/32: stack protector: change the canary value per task

From: Christophe Leroy
Date: Mon Sep 17 2018 - 08:15:17 EST

Partially copied from commit df0698be14c66 ("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 powerpc, 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

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
I would have liked to use -mstack-protector-guard=tls -mstack-protector-guard-reg=r2
-mstack-protector-guard-offset=offsetof(struct task_struct, stack_canary) but I have
not found how set the value of offsetof(struct task_struct, stack_canary) in Makefile.
Any idea ?

arch/powerpc/kernel/asm-offsets.c | 3 +++
arch/powerpc/kernel/entry_32.S | 5 +++++
2 files changed, 8 insertions(+)

diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 89cf15566c4e..cb02d23764ca 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -89,6 +89,9 @@ int main(void)
DEFINE(THREAD_INFO_GAP, _ALIGN_UP(sizeof(struct thread_info), 16));
OFFSET(KSP_LIMIT, thread_struct, ksp_limit);
#endif /* CONFIG_PPC64 */
+ DEFINE(TSK_STACK_CANARY, offsetof(struct task_struct, stack_canary));

OFFSET(TI_livepatch_sp, thread_info, livepatch_sp);
diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index e58c3f467db5..0cdb4170a21d 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -721,6 +721,11 @@ BEGIN_FTR_SECTION
mtspr SPRN_SPEFSCR,r0 /* restore SPEFSCR reg */
#endif /* CONFIG_SPE */
+#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
+ lwz r0, TSK_STACK_CANARY(r2)
+ lis r4, __stack_chk_guard@ha
+ stw r0, __stack_chk_guard@l(r4)

lwz r0,_CCR(r1)
mtcrf 0xFF,r0