[PATCH v8 02/12] x86_64: load_percpu_segment: read irq_stack_union.gs_base before load_segment

[Andrey Ryabinin]

Reading irq_stack_union.gs_base after load_segment creates troubles for kasan.
Compiler inserts __asan_load in between load_segment and wrmsrl. If kernel
built with stackprotector this will result in boot failure because __asan_load
has stackprotector.

To avoid this irq_stack_union.gs_base stored to temporary variable before
load_segment, so __asan_load will be called before load_segment().

There are two alternative ways to fix this:
 a) Add __attribute__((no_sanitize_address)) to load_percpu_segment(),
    which tells compiler to not instrument this function. However this
    will result in build failure with CONFIG_KASAN=y and CONFIG_OPTIMIZE_INLINING=y.

 b) Add -fno-stack-protector for mm/kasan/kasan.c

Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
---
 arch/x86/kernel/cpu/common.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 5475f67..1291d69 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -391,8 +391,10 @@ void load_percpu_segment(int cpu)
 #ifdef CONFIG_X86_32
 	loadsegment(fs, __KERNEL_PERCPU);
 #else
+	void *gs_base = per_cpu(irq_stack_union.gs_base, cpu);
+
 	loadsegment(gs, 0);
-	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
+	wrmsrl(MSR_GS_BASE, (unsigned long)gs_base);
 #endif
 	load_stack_canary_segment();
 }
-- 
2.1.3

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