[PATCH v4 1/2] powerpc/32: add stack protector support

From: Christophe Leroy
Date: Wed Sep 26 2018 - 07:40:43 EST

This functionality was tentatively added in the past
(commit 6533b7c16ee5 ("powerpc: Initial stack protector
(-fstack-protector) support")) but had to be reverted
(commit f2574030b0e3 ("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@xxxxxx>
---
v4: disable stack protector in bootx_init

v3: the offset is now defined by a rule in the Makefile. No need anymore
to take stack_canary out of the randomised area of task_struct

arch/powerpc/Kconfig | 1 +
arch/powerpc/Makefile | 10 +++++++++
arch/powerpc/include/asm/stackprotector.h | 34 +++++++++++++++++++++++++++++++
arch/powerpc/kernel/Makefile | 2 ++
arch/powerpc/kernel/asm-offsets.c | 3 +++
arch/powerpc/platforms/powermac/Makefile | 1 +
6 files changed, 51 insertions(+)
create mode 100644 arch/powerpc/include/asm/stackprotector.h

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a80669209155..3bcb05929931 100644
--- a/arch/powerpc/Kconfig
+++ b/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 $(cc-option,-mstack-protector-guard=tls) && PPC32
select HAVE_CONTEXT_TRACKING if PPC64
select HAVE_DEBUG_KMEMLEAK
select HAVE_DEBUG_STACKOVERFLOW
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 07d9dce7eda6..45b8eb4d8fe7 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -112,6 +112,9 @@ KBUILD_LDFLAGS += -m elf$(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
diff --git a/arch/powerpc/include/asm/stackprotector.h b/arch/powerpc/include/asm/stackprotector.h
new file mode 100644
index 000000000000..263e2aab1862
--- /dev/null
+++ b/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>
+#include <asm/paca.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. */
+ get_random_bytes(&canary, sizeof(canary));
+ canary ^= mftb();
+ canary ^= LINUX_VERSION_CODE;
+
+ current->stack_canary = canary;
+}
+
+#endif /* _ASM_STACKPROTECTOR_H */
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 1e64cfe22a83..85ffa488dfb5 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/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)
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index ba9d0fc98730..a992f7a53cf3 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/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);
diff --git a/arch/powerpc/platforms/powermac/Makefile b/arch/powerpc/platforms/powermac/Makefile
index 561a67d65e4d..923bfb340433 100644
--- a/arch/powerpc/platforms/powermac/Makefile
+++ b/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
--
2.13.3