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/Kconfig

@@ -180,7 +180,7 @@ config PPC
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_CBPF_JIT			if !PPC64
-	select HAVE_STACKPROTECTOR		if $(cc-option,-mstack-protector-guard=tls) && PPC32
+	select HAVE_STACKPROTECTOR		if $(cc-option,-mstack-protector-guard=tls)
 	select HAVE_CONTEXT_TRACKING		if PPC64
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DEBUG_STACKOVERFLOW

arch/powerpc/Makefile

@@ -113,7 +113,11 @@ KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
 endif
 
 cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
+ifdef CONFIG_PPC64
+cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r13
+else
 cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
+endif
 
 LDFLAGS_vmlinux-y := -Bstatic
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
@@ -411,8 +415,12 @@ ifdef CONFIG_STACKPROTECTOR
 prepare: stack_protector_prepare
 
 stack_protector_prepare: prepare0
+ifdef CONFIG_PPC64
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "PACA_CANARY") print $$3;}' include/generated/asm-offsets.h))
+else
 	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
 endif
+endif
 
 # Use the file '.tmp_gas_check' for binutils tests, as gas won't output
 # to stdout and these checks are run even on install targets.

arch/powerpc/include/asm/paca.h

@@ -256,6 +256,9 @@ struct paca_struct {
 	struct slb_entry *mce_faulty_slbs;
 	u16 slb_save_cache_ptr;
 #endif /* CONFIG_PPC_BOOK3S_64 */
+#ifdef CONFIG_STACKPROTECTOR
+	unsigned long canary;
+#endif
 } ____cacheline_aligned;
 
 extern void copy_mm_to_paca(struct mm_struct *mm);

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 */

arch/powerpc/kernel/asm-offsets.c

@@ -81,6 +81,9 @@ int main(void)
 	OFFSET(MM, task_struct, mm);
 #ifdef CONFIG_STACKPROTECTOR
 	OFFSET(TASK_CANARY, task_struct, stack_canary);
+#ifdef CONFIG_PPC64
+	OFFSET(PACA_CANARY, paca_struct, canary);
+#endif
 #endif
 	OFFSET(MMCONTEXTID, mm_struct, context.id);
 #ifdef CONFIG_PPC64

arch/powerpc/kernel/entry_64.S

@@ -624,6 +624,10 @@ _GLOBAL(_switch)
 
 	addi	r6,r4,-THREAD	/* Convert THREAD to 'current' */
 	std	r6,PACACURRENT(r13)	/* Set new 'current' */
+#if defined(CONFIG_STACKPROTECTOR)
+	ld	r6, TASK_CANARY(r6)
+	std	r6, PACA_CANARY(r13)
+#endif
 
 	ld	r8,KSP(r4)	/* new stack pointer */
 #ifdef CONFIG_PPC_BOOK3S_64