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

[Jürgen Groß]

On 24.08.20 17:58, Andrew Cooper wrote:
> On 24/08/2020 16:21, peterz@xxxxxxxxxxxxx wrote:
> > On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
> > > On 24/08/2020 11:14, peterz@xxxxxxxxxxxxx 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) <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 +18,15 @@ 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?
>
> Dealers choice, but this option would be slightly better overall.
>
> (Are there any other cases where Linux will be running in Ring 3?  I
> haven't been paying attention to recent changes in PVOps.)

I'm not aware of any other case running kernel code in ring 3.


Juergen