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    Subject[PATCH 5.4 016/261] x86,smap: Fix smap_{save,restore}() alternatives
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    [ Upstream commit 1ff865e343c2b59469d7e41d370a980a3f972c71 ]

    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
    when it hits between the PUSH and POP instructions.

    Rewrite the functions to only have a conditional jump.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Miroslav Benes <mbenes@suse.cz>
    Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Link: https://lkml.kernel.org/r/20200429101802.GI13592@hirez.programming.kicks-ass.net
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/include/asm/smap.h | 11 ++++++++---
    1 file changed, 8 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
    index 27c47d183f4b..8b58d6975d5d 100644
    --- a/arch/x86/include/asm/smap.h
    +++ b/arch/x86/include/asm/smap.h
    @@ -57,8 +57,10 @@ static __always_inline unsigned long smap_save(void)
    {
    unsigned long flags;

    - asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
    - X86_FEATURE_SMAP)
    + asm volatile ("# smap_save\n\t"
    + ALTERNATIVE("jmp 1f", "", X86_FEATURE_SMAP)
    + "pushf; pop %0; " __ASM_CLAC "\n\t"
    + "1:"
    : "=rm" (flags) : : "memory", "cc");

    return flags;
    @@ -66,7 +68,10 @@ static __always_inline unsigned long smap_save(void)

    static __always_inline void smap_restore(unsigned long flags)
    {
    - asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
    + asm volatile ("# smap_restore\n\t"
    + ALTERNATIVE("jmp 1f", "", X86_FEATURE_SMAP)
    + "push %0; popf\n\t"
    + "1:"
    : : "g" (flags) : "memory", "cc");
    }

    --
    2.25.1


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