powerpc/64: add stack protector support

On PPC64, as register r13 points to the paca_struct at all time,
this patch adds a copy of the canary there, which is copied at
task_switch.
That new canary is then used by using the following GCC options:
-mstack-protector-guard=tls
-mstack-protector-guard-reg=r13
-mstack-protector-guard-offset=offsetof(struct paca_struct, canary))

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/stackprotector.h

@@ -11,6 +11,7 @@
 #include <linux/version.h>
 #include <asm/reg.h>
 #include <asm/current.h>
+#include <asm/paca.h>
 
 /*
  * Initialize the stackprotector canary value.
@@ -29,6 +30,9 @@ static __always_inline void boot_init_stack_canary(void)
 	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
+#ifdef CONFIG_PPC64
+	get_paca()->canary = canary;
+#endif
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */