From: Mike Galbraith <efault@gmx.de>
To: Kees Cook <keescook@chromium.org>,
"David S. Miller" <davem@davemloft.net>,
Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
"Reshetova, Elena" <elena.reshetova@intel.com>,
Network Development <netdev@vger.kernel.org>
Subject: Re: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection
Date: Thu, 31 Aug 2017 06:38:18 +0200 [thread overview]
Message-ID: <1504154298.23109.23.camel@gmx.de> (raw)
In-Reply-To: <CAGXu5jKYPp5D+YxtqrJXsi48UUbjfeREngsys-dU1eP4RnnL+w@mail.gmail.com>
On Wed, 2017-08-30 at 21:10 -0700, Kees Cook wrote:
> On Wed, Aug 30, 2017 at 9:01 PM, Kees Cook <keescook@chromium.org> wrote:
> > On Wed, Aug 30, 2017 at 8:12 PM, Mike Galbraith <efault@gmx.de> wrote:
> >> On Wed, 2017-08-30 at 19:27 -0700, Kees Cook wrote:
> >>
> >>> Interesting! Can you try with 633547973ffc3 ("net: convert
> >>> sk_buff.users from atomic_t to refcount_t") reverted? I'll see if
> >>> running haveged will help me trigger this on my system...
> >>
> >> With that (plus 230cd1279d001 fix to it) reverted, vbox boots.
> >
> > Wonderful! Thank you so much for helping track this down.
> >
> > So, it seems that sk_buff.users will need some more special attention
> > before we can convert it to refcount.
> >
> > x86-refcount will saturate with refcount_dec_and_test() if the result
> > is negative. But that would mean at least starting at 0. FULL should
> > have WARNed in this case, so I remain slightly confused why it was
> > missed by FULL.
>
> Actually, if this is a race condition it's possible that FULL is slow
> enough to miss it...
>
> I bet something briefly takes the refcount negative, and with
> unchecked atomics, it come back up positive again during the race.
> FULL may miss the race, and x86-refcount will catch it and saturate...
Hm, I'll go have a stare.. not that that's likely to turn anything up,
memory ordering stares usually inducing a zombie like state.
-Mike
next prev parent reply other threads:[~2017-08-31 4:39 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
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[not found] ` <1504025721.6024.25.camel@gmx.de>
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[not found] ` <1504149176.23109.9.camel@gmx.de>
2017-08-31 4:01 ` tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection Kees Cook
2017-08-31 4:10 ` Kees Cook
2017-08-31 4:38 ` Mike Galbraith [this message]
2017-08-31 13:58 ` Mike Galbraith
2017-08-31 17:00 ` Kees Cook
2017-08-31 17:19 ` Mike Galbraith
2017-08-31 18:45 ` Kees Cook
2017-09-01 6:57 ` Mike Galbraith
2017-09-01 13:09 ` Mike Galbraith
2017-09-01 17:12 ` Kees Cook
2017-09-01 17:52 ` Mike Galbraith
2017-09-01 18:58 ` Kees Cook
2017-09-01 19:24 ` Mike Galbraith
2017-09-01 19:40 ` Kees Cook
2017-08-31 19:28 ` Kees Cook
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