Re: [PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives

[Brian Gerst]

On Tue, Apr 28, 2020 at 3:21 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx>
> ---
>  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");
>  }

POPF is an expensive instruction that should be avoided if possible.
A better solution would be to have the alternative jump over the
push/pop when SMAP is disabled.

--
Brian Gerst