linux-kernel - Re: [PATCH] x86/entry: Fix AC assertion

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Date:   Mon, 24 Aug 2020 17:21:35 +0200
From:   peterz@...radead.org
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     <x86@...nel.org>, Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Andy Lutomirski" <luto@...nel.org>
Subject: Re: [PATCH] x86/entry: Fix AC assertion

On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
> On 24/08/2020 11:14, peterz@...radead.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) <peterz@...radead.org>
> > Acked-by: Andy Lutomirski <luto@...nel.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?