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Subject: Re: KASAN: use-after-free Read in unregister_shrinker
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     "J. Bruce Fields" <bfields@fieldses.org>,
        syzbot <syzbot+83a43746cebef3508b49@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, bfields@redhat.com,
        chris@chrisdown.name, Daniel Jordan <daniel.m.jordan@oracle.com>,
        guro@fb.com, Johannes Weiner <hannes@cmpxchg.org>,
        Jeff Layton <jlayton@kernel.org>, laoar.shao@gmail.com,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, linux-nfs@vger.kernel.org,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        yang.shi@linux.alibaba.com
References: <0000000000005a4b99058a97f42e@google.com>
 <b67a0f5d-c508-48a7-7643-b4251c749985@virtuozzo.com>
 <20190606131334.GA24822@fieldses.org>
 <275f77ad-1962-6a60-e60b-6b8845f12c34@virtuozzo.com>
 <CACT4Y+aJQ6J5WdviD+cOmDoHt2Dj=Q4uZ4vHbCfHe+_TCEY6-Q@mail.gmail.com>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <00ec828a-0dcb-ca70-e938-ca26a6a8b675@virtuozzo.com>
Date:   Thu, 6 Jun 2019 17:54:19 +0300
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On 06.06.2019 17:40, Dmitry Vyukov wrote:
> On Thu, Jun 6, 2019 at 3:43 PM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>> On 06.06.2019 16:13, J. Bruce Fields wrote:
>>> On Thu, Jun 06, 2019 at 10:47:43AM +0300, Kirill Tkhai wrote:
>>>> This may be connected with that shrinker unregistering is forgotten on error path.
>>>
>>> I was wondering about that too.  Seems like it would be hard to hit
>>> reproduceably though: one of the later allocations would have to fail,
>>> then later you'd have to create another namespace and this time have a
>>> later module's init fail.
>>
>> Yes, it's had to bump into this in real life.
>>
>> AFAIU, syzbot triggers such the problem by using fault-injections
>> on allocation places should_failslab()->should_fail(). It's possible
>> to configure a specific slab, so the allocations will fail with
>> requested probability.
> 
> No fault injection was involved in triggering of this bug.
> Fault injection is clearly visible in console log as "INJECTING
> FAILURE at this stack track" splats and also for bugs with repros it
> would be noted in the syzkaller repro as "fault_call": N. So somehow
> this bug was triggered as is.
> 
> But overall syzkaller can do better then the old probabilistic
> injection. The probabilistic injection tend to both under-test what we
> want to test and also crash some system services. syzkaller uses the
> new "systematic fault injection" that allows to test specifically each
> failure site separately in each syscall separately.

Oho! Interesting.

> All kernel testing systems should use it. Also in couple with KASAN,
> KMEMLEAK, LOCKDEP. It's indispensable in finding kernel bugs.