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

[peterz]

On Wed, Sep 02, 2020 at 06:24:27PM +0200, Jürgen Groß wrote:
> 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.

Correct, this asserts that 64bit Xen-PV will never have AC set; it had
better not have it set since it runs in ring 3.