[PATCHv2] arm64: kasan: clear stale stack poison

[Mark Rutland]

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 <mark.rutland at arm.com>
Cc: Alexander Potapenko <glider at google.com>
Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
---
 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(-)

Since v1 [1]:
* Remove unneeded offset
* Simplify calculation

For the timebeing I've retained the arm64-specific hotplug fix, though it might
be possible to handle the hotplug case in common code (e.g. [2]). 

Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/409466.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/412923.html

diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index 2774fa3..6f00b76 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, from the lowest stack address in the
+	 * thread_union up to the covering up to the current stack pointer.
+	 * Shadow poison above this is preserved.
+	 */
+	.macro kasan_unpoison_stack
+	mov	x1, sp
+	and	x0, x1, #~(THREAD_SIZE - 1)
+	add	x0, x0, #(THREAD_INFO_SIZE)
+	sub	x1, x1, x0
+	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
+	.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 917d981..35ae2cb 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>
@@ -616,6 +617,7 @@ 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..3ad7681 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
 	mov	x0, #0			// return zero on success
 	ldp	x19, x20, [sp, #16]
 	ldp	x21, x22, [sp, #32]
-- 
1.9.1