linux-kernel - Re: general protection fault in show

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Message-ID: <alpine.DEB.2.20.1712151735120.1702@nanos>
Date:   Fri, 15 Dec 2017 17:40:15 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Eric Biggers <ebiggers3@...il.com>
cc:     Dmitry Vyukov <dvyukov@...gle.com>, Greg KH <greg@...ah.com>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>, security@...nel.org,
        Akinobu Mita <akinobu.mita@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller-bugs@...glegroups.com,
        Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: general protection fault in show_timer

On Thu, 14 Dec 2017, Eric Biggers wrote:
> On Thu, Dec 14, 2017 at 04:34:12PM +0100, Thomas Gleixner wrote:
> > On Thu, 14 Dec 2017, Dmitry Vyukov wrote:
> > > On Thu, Dec 14, 2017 at 3:26 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > > > On Tue, 12 Dec 2017, Greg KH wrote:
> > > >> Did this ever go anywhere?  I don't see it in Linus's tree yet...
> > > >
> > > > I learned yesterday that syzboz is understuffed and cannot test patches, so
> > > > I need to find a minute to run the reproducer myself and verify that the
> > > > patch is correct.
> > > 
> > > Hi Thomas,
> > > 
> > > Why do you say so? Have you tried to ask it to test?
> > > https://github.com/google/syzkaller/blob/master/docs/syzbot.md#communication-with-syzbot
> > > What happened?
> > 
> > Eric explained that to me yesterday and I did not try yet. 
> > 
> 
> Your patch definitely fixes the bug (I tested the C reproducers, you just need
> to build a kernel with CONFIG_CHECKPOINT_RESTORE=y and CONFIG_POSIX_TIMERS=y,
> then run them).  The real question is whether the check being introduced is too
> strict -- are there users passing in other values for ->sigev_notify that would
> be broken?  That I can't really answer.

Me neither. The manpage is rather clear about the possible values, so I
don't expect wreckage. Aside of that non canonical values would have to
have bit 2, i.e. SIGEV_THREAD_ID cleared because that already has a
restriction that it's only allowed with SIGEV_SIGNAL. So unlikely...

If really some crap application breaks we can handle it in the default
clause by setting it to SIGEV_SIGNAL. Though I rather prefer not to do that
unless it turns out to be absolutely necessary.

Thanks,

	tglx