linux-kernel - Re: [PATCH 3/3] x86/mm, tracing: Fix CR2 corruption

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Date:   Wed, 3 Jul 2019 15:26:47 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        He Zhe <zhe.he@...driver.com>,
        Joel Fernandes <joel@...lfernandes.org>, devel@...ukata.com
Subject: Re: [PATCH 3/3] x86/mm, tracing: Fix CR2 corruption

On Wed, Jul 3, 2019 at 3:01 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Jul 03, 2019 at 01:27:09PM -0700, Andy Lutomirski wrote:
> > On Wed, Jul 3, 2019 at 3:28 AM root <peterz@...radead.org> wrote:
>
> > > @@ -1338,18 +1347,9 @@ ENTRY(error_entry)
> > >         movq    %rax, %rsp                      /* switch stack */
> > >         ENCODE_FRAME_POINTER
> > >         pushq   %r12
> > > -
> > > -       /*
> > > -        * We need to tell lockdep that IRQs are off.  We can't do this until
> > > -        * we fix gsbase, and we should do it before enter_from_user_mode
> > > -        * (which can take locks).
> > > -        */
> > > -       TRACE_IRQS_OFF
> >
> > This hunk looks wrong.  Am I missing some other place that handles the
> > case where we enter from kernel mode and IRQs were on?
>
> > > -   CALL_enter_from_user_mode
> > >     ret
> > >
> > >  .Lerror_entry_done:
> > > -   TRACE_IRQS_OFF
> > >     ret
> > >
> > >     /*
>
> Did you perchance mean to complain about the .Lerror_entry_done one?

Yes, duh.

I would suggest compiling with DEBUG_LOCKDEP and running perf and the
x86 selftests.  IIRC that's what catches most of the IRQ flag tracing
errors.

>
> Because I'm not seeing how the one before CALL_enter_from_user_mode can
> ever be from-kernel.
>
> But yes, that .Lerror_entry_done one looks fishy.