linux-kernel - Re: kvm: use-after-free in process

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Date:   Sun, 11 Dec 2016 09:40:29 +0100
From:   Vegard Nossum <vegard.nossum@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Steve Rutherford <srutherford@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: kvm: use-after-free in process_srcu

On 11 December 2016 at 07:46, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> Hello,
>
> I am getting the following use-after-free reports while running
> syzkaller fuzzer.
> On commit 318c8932ddec5c1c26a4af0f3c053784841c598e (Dec 7).
> Unfortunately it is not reproducible, but all reports look sane and
> very similar, so I would assume that it is some hard to trigger race.
> In all cases the use-after-free offset within struct kvm is 344 bytes.
> This points to srcu field, which starts at 208 with size 360 (I have
> some debug configs enabled).
[...]
>  [  376.024345] [<ffffffff81a77f7e>] __fput+0x34e/0x910 fs/file_table.c:208
>  [  376.024345] [<ffffffff81a785ca>] ____fput+0x1a/0x20 fs/file_table.c:244

I've been hitting what I think is a struct file refcounting bug which
causes similar symptoms as you have here (the struct file is freed
while somebody still has an active reference to it).

>  [  376.024345] [<ffffffff81483c20>] task_work_run+0x1a0/0x280
> kernel/task_work.c:116
>  [  376.024345] [<     inline     >] exit_task_work include/linux/task_work.h:21
>  [  376.024345] [<ffffffff814129e2>] do_exit+0x1842/0x2650 kernel/exit.c:828
>  [  376.024345] [<ffffffff814139ae>] do_group_exit+0x14e/0x420 kernel/exit.c:932
>  [  376.024345] [<ffffffff81442b43>] get_signal+0x663/0x1880
> kernel/signal.c:2307
>  [  376.024345] [<ffffffff81239b45>] do_signal+0xc5/0x2190
> arch/x86/kernel/signal.c:807

Was this or any other process by any chance killed by the OOM killer?
That seems to be a pattern in the crashes I've seen. If not, do you
know what killed this process?


Vegard