Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH] x86/entry: Fix AC assertion | From | Andrew Cooper <> | Date | Mon, 24 Aug 2020 15:22:06 +0100 |
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On 24/08/2020 11:14, peterz@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.
> > Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks") > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Acked-by: Andy Lutomirski <luto@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.
~Andrew
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