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

[Jürgen Groß]

On 02.09.20 17:58, Brian Gerst wrote:
> 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.

Right, and this test will nevertheless result in setting AC in the mask.
IIRC this was the main objective here.


Juergen