On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
> On 24/08/2020 11:14, pet...@infradead.org wrote:
> > 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.
>
> Technically, you don't patch in, rather than patch out.
True.
> > 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 +18,15 @@ 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))
> > + mask |= X86_EFLAGS_AC;
>
> The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
> actually running in Ring 3, and therefore susceptible to #AC's which
> wouldn't occur natively.
So do you then want it to be something like:
if (boot_cpu_has(X86_FEATURE_SMAP) ||
(IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
? Or are you fine with the proposed?
