powerpc/32: Change the stack protector canary value per task

Partially copied from commit df0698be ("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
useful.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/asm-offsets.c

@@ -91,6 +91,9 @@ int main(void)
 	DEFINE(TI_livepatch_sp, offsetof(struct thread_info, livepatch_sp));
 #endif
 
+#ifdef CONFIG_CC_STACKPROTECTOR
+	DEFINE(TSK_STACK_CANARY, offsetof(struct task_struct, stack_canary));
+#endif
 	DEFINE(KSP, offsetof(struct thread_struct, ksp));
 	DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
 #ifdef CONFIG_BOOKE

arch/powerpc/kernel/entry_32.S

@@ -674,7 +674,11 @@ BEGIN_FTR_SECTION
 	mtspr	SPRN_SPEFSCR,r0		/* restore SPEFSCR reg */
 END_FTR_SECTION_IFSET(CPU_FTR_SPE)
 #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)
+#endif
 	lwz	r0,_CCR(r1)
 	mtcrf	0xFF,r0
 	/* r3-r12 are destroyed -- Cort */