linux-kernel - Re: [tip: x86/urgent] x86/entry, selftests: Further improve user entry sanity checks

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Date:   Sat, 22 Aug 2020 14:59:37 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86 <x86@...nel.org>
Subject: Re: [tip: x86/urgent] x86/entry, selftests: Further improve user
 entry sanity checks

On Thu, Aug 20, 2020 at 3:24 AM <peterz@...radead.org> wrote:
>
> On Sat, Jul 04, 2020 at 05:49:10PM -0000, tip-bot2 for Andy Lutomirski wrote:
>
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index f392a8b..e83b3f1 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -49,6 +49,23 @@
> >  static void check_user_regs(struct pt_regs *regs)
> >  {
> >       if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
> > +             /*
> > +              * Make sure that the entry code gave us a sensible EFLAGS
> > +              * register.  Native because we want to check the actual CPU
> > +              * 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));
>
> This triggers with AC|TF on my !SMAP enabled machine.
>
> something like so then?
>
> diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
> index a8f9315b9eae..76410964585f 100644
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -18,8 +18,15 @@ static __always_inline void arch_check_user_regs(struct pt_regs *regs)
>                  * 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))
> +                       mask |= X86_EFLAGS_AC;
> +
> +               WARN_ON_ONCE(flags & mask);
>
>                 /* We think we came from user mode. Make sure pt_regs agrees. */
>                 WARN_ON_ONCE(!user_mode(regs));

LGTM.

Acked-by: Andy Lutomirski <luto@...nel.org>