Re: [PATCH 01/13] x86/entry: Fix AC assertion

[Brian Gerst]

On Wed, Sep 2, 2020 at 9:38 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
> improve user entry sanity checks") unconditionally triggers on my IVB
> machine because it does not support SMAP.
>
> For !SMAP hardware we patch out CLAC/STAC instructions and thus if
> userspace sets AC, we'll still have it set after entry.
>
> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/entry-common.h |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -18,8 +18,16 @@ static __always_inline void arch_check_u
>                  * state, not the interrupt state as imagined by Xen.
>                  */
>                 unsigned long flags = native_save_fl();
> -               WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
> -                                     X86_EFLAGS_NT));
> +               unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
> +
> +               /*
> +                * For !SMAP hardware we patch out CLAC on entry.
> +                */
> +               if (boot_cpu_has(X86_FEATURE_SMAP) ||
> +                   (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
> +                       mask |= X86_EFLAGS_AC;

Is the explicit Xen check necessary?  IIRC the Xen hypervisor will
filter out the SMAP bit in the cpuid pvop.

--
Brian Gerst