Messages in this thread Patch in this message | | | Date | Tue, 28 Apr 2020 21:11:04 +0200 | From | Peter Zijlstra <> | Subject | [PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives |
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As reported by objtool:
lib/ubsan.o: warning: objtool: .altinstr_replacement+0x0: alternative modifies stack lib/ubsan.o: warning: objtool: .altinstr_replacement+0x7: alternative modifies stack
the smap_{save,restore}() alternatives violate (the newly enforced) rule on stack invariance. That is, due to there only being a single ORC table it must be valid to any alternative. These alternatives violate this with the direct result that unwinds will not be correct in between these calls.
[ In specific, since we force SMAP on for objtool, running on !SMAP hardware will observe a different stack-layout and the ORC unwinder will stumble. ]
So rewrite the functions to unconditionally save/restore the flags, which gives an invariant stack layout irrespective of the SMAP state.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/include/asm/smap.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
--- a/arch/x86/include/asm/smap.h +++ b/arch/x86/include/asm/smap.h @@ -57,16 +57,19 @@ static __always_inline unsigned long sma { unsigned long flags; - asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC, - X86_FEATURE_SMAP) - : "=rm" (flags) : : "memory", "cc"); + asm volatile ("# smap_save\n\t" + "pushf; pop %0" + : "=rm" (flags) : : "memory"); + + clac(); return flags; } static __always_inline void smap_restore(unsigned long flags) { - asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP) + asm volatile ("# smap_restore\n\t" + "push %0; popf" : : "g" (flags) : "memory", "cc"); }
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