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 <pet...@infradead.org> wrote:
> > >
> > > From: Peter Zijlstra <pet...@infradead.org>
> > >
> > > 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) <pet...@infradead.org>
> > > Acked-by: Andy Lutomirski <l...@kernel.org>
> > > ---
> > > 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.
