This patch is a followup to the discussion in [1]. When using KASAN and CPU idle and/or CPU hotplug, KASAN leaves the stack shadow poisoned on exit from the kernel, and this poison is later hit when a CPU is brought online and reuses that portion of the stack. Hitting the poison depends on stackframe layout, so the bug only manifests in some configurations.
I think that the hotplug issue is generic, and x86 is affected. I couldn't spot magic around idle, so x86 may be fine there. It would be great if someone familiar with the x86 code could prove/disprove either of those assertions. If x86 is affected, it likely makes sense to unpoison the stack in common code prior to bringing a CPU online to avoid that. For idle I'm not keen on having to perform a memset of THREAD_SIZE/8 every time a CPU re-enters the kernel. I don't yet have numbers for how bad that is, but it doesn't sound good. Thanks, Mark. [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/408961.html ---->8---- When a CPU is shut down or placed into a low power state, the functions on the critical path to firmware never return, and hence their epilogues never execute. When using KASAN, this means that the shadow entries for the corresponding stack are poisoned but never unpoisoned. When a CPU subsequently re-enters the kernel via another path, and begins using the stack, it may hit stale poison values, leading to false-positive KASAN failures. We can't ensure that all functions on the critical path are not instrumented. For CPU hotplug this includes lots of core code starting from secondary_start_kernel, and for CPU idle we can't ensure that specific functions are not instrumented, as the compiler always poisons the stack even when told to not instrument a function: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69863 This patch works around the issue by forcefully unpoisoning the shadow region for all stack on the critical path, before we return to instrumented C code. As we cannot statically determine the stack usage of code in the critical path, we must clear the shadow for all remaining stack, meaning that we must clear up to 2K of shadow memory each time a CPU enters the kernel from idle or hotplug. Signed-off-by: Mark Rutland <[email protected]> Cc: Andrey Ryabinin <[email protected]> Cc: Ard Biesheuvel <[email protected]> Cc: Catalin Marinas <[email protected]> Cc: Lorenzo Pieralisi <[email protected]> Cc: Will Deacon <[email protected]> --- arch/arm64/include/asm/kasan.h | 40 ++++++++++++++++++++++++++++++++++------ arch/arm64/kernel/asm-offsets.c | 1 + arch/arm64/kernel/head.S | 2 ++ arch/arm64/kernel/sleep.S | 2 ++ 4 files changed, 39 insertions(+), 6 deletions(-) diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h index 2774fa3..b75b171 100644 --- a/arch/arm64/include/asm/kasan.h +++ b/arch/arm64/include/asm/kasan.h @@ -1,10 +1,30 @@ #ifndef __ASM_KASAN_H #define __ASM_KASAN_H -#ifndef __ASSEMBLY__ - +#ifndef LINKER_SCRIPT #ifdef CONFIG_KASAN +#ifdef __ASSEMBLY__ + +#include <asm/asm-offsets.h> +#include <asm/thread_info.h> + + /* + * Remove stale shadow posion for the stack left over from a prior + * hot-unplug or idle exit, covering up to offset bytes above the + * current stack pointer. Shadow poison above this is preserved. + */ + .macro kasan_unpoison_stack offset=0 + add x1, sp, #\offset + and x0, x1, #~(THREAD_SIZE - 1) + add x0, x0, #THREAD_INFO_SIZE + and x1, x1, #(THREAD_SIZE - 1) + sub x1, x1, #THREAD_INFO_SIZE + bl kasan_unpoison_shadow + .endm + +#else /* __ASSEMBLY__ */ + #include <linux/linkage.h> #include <asm/memory.h> @@ -30,9 +50,17 @@ void kasan_init(void); asmlinkage void kasan_early_init(void); -#else +#endif /* __ASSEMBLY__ */ + +#else /* CONFIG_KASAN */ + +#ifdef __ASSEMBLY__ + .macro kasan_unpoison_stack offset + .endm +#else /* __ASSEMBLY */ static inline void kasan_init(void) { } -#endif +#endif /* __ASSEMBLY__ */ -#endif -#endif +#endif /* CONFIG_KASAN */ +#endif /* LINKER_SCRIPT */ +#endif /* __ASM_KASAN_H */ diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c index fffa4ac6..c615fa3 100644 --- a/arch/arm64/kernel/asm-offsets.c +++ b/arch/arm64/kernel/asm-offsets.c @@ -39,6 +39,7 @@ int main(void) DEFINE(TI_ADDR_LIMIT, offsetof(struct thread_info, addr_limit)); DEFINE(TI_TASK, offsetof(struct thread_info, task)); DEFINE(TI_CPU, offsetof(struct thread_info, cpu)); + DEFINE(THREAD_INFO_SIZE, sizeof(struct thread_info)); BLANK(); DEFINE(THREAD_CPU_CONTEXT, offsetof(struct task_struct, thread.cpu_context)); BLANK(); diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S index ffe9c2b..a0c3ec7 100644 --- a/arch/arm64/kernel/head.S +++ b/arch/arm64/kernel/head.S @@ -29,6 +29,7 @@ #include <asm/asm-offsets.h> #include <asm/cache.h> #include <asm/cputype.h> +#include <asm/kasan.h> #include <asm/kernel-pgtable.h> #include <asm/memory.h> #include <asm/pgtable-hwdef.h> @@ -611,6 +612,7 @@ ENTRY(__secondary_switched) and x0, x0, #~(THREAD_SIZE - 1) msr sp_el0, x0 // save thread_info mov x29, #0 + kasan_unpoison_stack b secondary_start_kernel ENDPROC(__secondary_switched) diff --git a/arch/arm64/kernel/sleep.S b/arch/arm64/kernel/sleep.S index e33fe33..3b95841 100644 --- a/arch/arm64/kernel/sleep.S +++ b/arch/arm64/kernel/sleep.S @@ -2,6 +2,7 @@ #include <linux/linkage.h> #include <asm/asm-offsets.h> #include <asm/assembler.h> +#include <asm/kasan.h> .text /* @@ -145,6 +146,7 @@ ENTRY(cpu_resume_mmu) ENDPROC(cpu_resume_mmu) .popsection cpu_resume_after_mmu: + kasan_unpoison_stack 96 mov x0, #0 // return zero on success ldp x19, x20, [sp, #16] ldp x21, x22, [sp, #32] -- 1.9.1
