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

[Brian Gerst]

On Wed, Apr 29, 2020 at 6:18 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 29, 2020 at 10:30:53AM +0200, Peter Zijlstra wrote:
> > > 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.
> >
> > Yeah. I think I had that, but then confused myself again. I don't think
> > it matters much if you look at where it's used though.
> >
> > Still, let me try the jmp thing again..
>
> Here goes..
>
> ---
> Subject: x86,smap: Fix smap_{save,restore}() alternatives
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Tue Apr 28 19:57:59 CEST 2020
>
> 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@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/smap.h |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -57,8 +57,10 @@ static __always_inline unsigned long sma
>  {
>         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 sma
>
>  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");
>  }
>

Looks good.  Alternatively, you could use static_cpu_has(X86_FEATURE_SMAP).

--
Brian Gerst