From: Eric Biggers <ebiggers3@gmail.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: syzbot <syzbot+bacbe5d8791f30c9cee5@syzkaller.appspotmail.com>,
akpm@linux-foundation.org, aneesh.kumar@linux.vnet.ibm.com,
dan.j.williams@intel.com, james.morse@arm.com,
kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, mingo@kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: possible deadlock in get_user_pages_unlocked
Date: Thu, 1 Feb 2018 21:35:02 -0800 [thread overview]
Message-ID: <20180202053502.GB949@zzz.localdomain> (raw)
In-Reply-To: <20180202045020.GF30522@ZenIV.linux.org.uk>
On Fri, Feb 02, 2018 at 04:50:20AM +0000, Al Viro wrote:
> On Thu, Feb 01, 2018 at 04:58:00PM -0800, syzbot wrote:
> > Hello,
> >
> > syzbot hit the following crash on upstream commit
> > 7109a04eae81c41ed529da9f3c48c3655ccea741 (Thu Feb 1 17:37:30 2018 +0000)
> > Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/ide
> >
> > So far this crash happened 2 times on upstream.
> > C reproducer is attached.
>
> Umm... How reproducible that is?
>
> > syzkaller reproducer is attached.
> > Raw console output is attached.
> > compiler: gcc (GCC) 7.1.1 20170620
> > .config is attached.
>
> Can't reproduce with gcc 5.4.1 (same .config, same C reproducer).
>
> It looks like __get_user_pages_locked() returning with *locked zeroed,
> but ->mmap_sem not dropped. I don't see what could've lead to it and
> attempts to reproduce had not succeeded so far...
>
> How long does it normally take for lockdep splat to trigger?
>
Try starting up multiple instances of the program; that sometimes helps with
these races that are hard to hit (since you may e.g. have a different number of
CPUs than syzbot used). If I start up 4 instances I see the lockdep splat after
around 2-5 seconds. This is on latest Linus tree (4bf772b1467). Also note the
reproducer uses KVM, so if you're running it in a VM it will only work if you've
enabled nested virtualization on the host (kvm_intel.nested=1).
Also it appears to go away if I revert ce53053ce378c21 ("kvm: switch
get_user_page_nowait() to get_user_pages_unlocked()").
- Eric
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next prev parent reply other threads:[~2018-02-02 5:35 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-02 0:58 possible deadlock in get_user_pages_unlocked syzbot
2018-02-02 4:50 ` Al Viro
2018-02-02 5:35 ` Eric Biggers [this message]
2018-02-02 5:46 ` Al Viro
2018-02-02 6:20 ` Al Viro
2018-02-02 8:57 ` Dmitry Vyukov
2018-02-10 1:36 ` Al Viro
2018-02-10 3:19 ` Eric Biggers
2018-03-10 4:15 ` Eric Biggers
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