powerpc/32: add stack protector support

This functionality was tentatively added in the past
(commit 6533b7c16e ("powerpc: Initial stack protector
(-fstack-protector) support")) but had to be reverted
(commit f2574030b0 ("powerpc: Revert the initial stack
protector support") because of GCC implementing it differently
whether it had been built with libc support or not.

Now, GCC offers the possibility to manually set the
stack-protector mode (global or tls) regardless of libc support.

This time, the patch selects HAVE_STACKPROTECTOR only if
-mstack-protector-guard=tls is supported by GCC.

On PPC32, as register r2 points to current task_struct at
all time, the stack_canary located inside task_struct can be
used directly by using the following GCC options:
-mstack-protector-guard=tls
-mstack-protector-guard-reg=r2
-mstack-protector-guard-offset=offsetof(struct task_struct, stack_canary))

The protector is disabled for prom_init and bootx_init as
it is too early to handle it properly.

 $ echo CORRUPT_STACK > /sys/kernel/debug/provoke-crash/DIRECT
[  134.943666] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: lkdtm_CORRUPT_STACK+0x64/0x64
[  134.943666]
[  134.955414] CPU: 0 PID: 283 Comm: sh Not tainted 4.18.0-s3k-dev-12143-ga3272be41209 #835
[  134.963380] Call Trace:
[  134.965860] [c6615d60] [c001f76c] panic+0x118/0x260 (unreliable)
[  134.971775] [c6615dc0] [c001f654] panic+0x0/0x260
[  134.976435] [c6615dd0] [c032c368] lkdtm_CORRUPT_STACK_STRONG+0x0/0x64
[  134.982769] [c6615e00] [ffffffff] 0xffffffff

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

arch/powerpc/Kconfig

@@ -180,6 +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_CONTEXT_TRACKING		if PPC64
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DEBUG_STACKOVERFLOW

arch/powerpc/Makefile

@@ -112,6 +112,9 @@ KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
 KBUILD_ARFLAGS	+= --target=elf$(BITS)-$(GNUTARGET)
 endif
 
+cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
+cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
+
 LDFLAGS_vmlinux-y := -Bstatic
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
 LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)
@@ -404,6 +407,13 @@ archclean:
 
 archprepare: checkbin
 
+ifdef CONFIG_STACKPROTECTOR
+prepare: stack_protector_prepare
+
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
+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.
 TOUT	:= .tmp_gas_check

arch/powerpc/include/asm/stackprotector.h

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * GCC stack protector support.
+ *
+ */
+
+#ifndef _ASM_STACKPROTECTOR_H
+#define _ASM_STACKPROTECTOR_H
+
+#include <linux/random.h>
+#include <linux/version.h>
+#include <asm/reg.h>
+#include <asm/current.h>
+
+/*
+ * Initialize the stackprotector canary value.
+ *
+ * NOTE: this must only be called from functions that never return,
+ * and it must always be inlined.
+ */
+static __always_inline void boot_init_stack_canary(void)
+{
+	unsigned long canary;
+
+	/* Try to get a semi random initial value. */
+	canary = get_random_canary();
+	canary ^= mftb();
+	canary ^= LINUX_VERSION_CODE;
+	canary &= CANARY_MASK;
+
+	current->stack_canary = canary;
+}
+
+#endif	/* _ASM_STACKPROTECTOR_H */

arch/powerpc/kernel/Makefile

@@ -20,6 +20,8 @@ CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 
+CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
+
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code
 CFLAGS_REMOVE_cputable.o = $(CC_FLAGS_FTRACE)

arch/powerpc/kernel/asm-offsets.c

@@ -79,6 +79,9 @@ int main(void)
 {
 	OFFSET(THREAD, task_struct, thread);
 	OFFSET(MM, task_struct, mm);
+#ifdef CONFIG_STACKPROTECTOR
+	OFFSET(TASK_CANARY, task_struct, stack_canary);
+#endif
 	OFFSET(MMCONTEXTID, mm_struct, context.id);
 #ifdef CONFIG_PPC64
 	DEFINE(SIGSEGV, SIGSEGV);

arch/powerpc/platforms/powermac/Makefile

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 CFLAGS_bootx_init.o  		+= -fPIC
+CFLAGS_bootx_init.o  		+= $(call cc-option, -fno-stack-protector)
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code