linux-kernel - Re: [PATCH 01/13] x86/entry: Fix AC assertion

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Message-ID: <20200902163115.GM1362448@hirez.programming.kicks-ass.net>
Date:   Wed, 2 Sep 2020 18:31:15 +0200
From:   peterz@...radead.org
To:     Jürgen Groß <jgross@...e.com>
Cc:     Brian Gerst <brgerst@...il.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Robert O'Callahan <rocallahan@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH 01/13] x86/entry: Fix AC assertion

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 <peterz@...radead.org> wrote:
> > > 
> > > From: Peter Zijlstra <peterz@...radead.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) <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 +18,16 @@ 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.