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chleroympe
chleroy
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mpe
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powerpc/32: add stack protector support
This functionality was tentatively added in the past (commit 6533b7c ("powerpc: Initial stack protector (-fstack-protector) support")) but had to be reverted (commit f257403 ("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 torvalds#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>
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‎arch/powerpc/Kconfig

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@@ -180,6 +180,7 @@ config PPC
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select HAVE_ARCH_SECCOMP_FILTER
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select HAVE_ARCH_TRACEHOOK
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select HAVE_CBPF_JIT if !PPC64
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select HAVE_STACKPROTECTOR if $(cc-option,-mstack-protector-guard=tls) && PPC32
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select HAVE_CONTEXT_TRACKING if PPC64
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select HAVE_DEBUG_KMEMLEAK
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select HAVE_DEBUG_STACKOVERFLOW

‎arch/powerpc/Makefile

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@@ -112,6 +112,9 @@ KBUILD_LDFLAGS += -m elf$(BITS)$(LDEMULATION)
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KBUILD_ARFLAGS += --target=elf$(BITS)-$(GNUTARGET)
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endif
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cflags-$(CONFIG_STACKPROTECTOR) += -mstack-protector-guard=tls
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cflags-$(CONFIG_STACKPROTECTOR) += -mstack-protector-guard-reg=r2
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LDFLAGS_vmlinux-y := -Bstatic
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LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
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LDFLAGS_vmlinux := $(LDFLAGS_vmlinux-y)
@@ -404,6 +407,13 @@ archclean:
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archprepare: checkbin
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ifdef CONFIG_STACKPROTECTOR
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prepare: stack_protector_prepare
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stack_protector_prepare: prepare0
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$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
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endif
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# Use the file '.tmp_gas_check' for binutils tests, as gas won't output
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# to stdout and these checks are run even on install targets.
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TOUT := .tmp_gas_check

‎arch/powerpc/include/asm/stackprotector.h

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

‎arch/powerpc/kernel/Makefile

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

‎arch/powerpc/kernel/asm-offsets.c

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

‎arch/powerpc/platforms/powermac/Makefile

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

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