Re: KASAN: use-after-free Read in percpu_ref_switch_to_atomic_rcu

Jann Horn
2020-03-06T14:58:26Z

+paulmck

On Wed, Mar 4, 2020 at 3:40 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 3/4/20 12:59 AM, Dmitry Vyukov wrote:
> > On Fri, Feb 7, 2020 at 9:14 AM syzbot
> > <syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com> wrote:
> >>
> >> Hello,
> >>
> >> syzbot found the following crash on:
> >>
> >> HEAD commit: 4c7d00cc Merge tag 'pwm/for-5.6-rc1' of git://git.kernel.o..
> >> git tree: upstream
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=12fec785e00000
> >> kernel config: https://syzkaller.appspot.com/x/.config?x=e162021ddededa72
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=e017e49c39ab484ac87a
> >> compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> >>
> >> Unfortunately, I don't have any reproducer for this crash yet.
> >>
> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >> Reported-by: syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com
> >
> > +io_uring maintainers
> >
> > Here is a repro:
> > https://gist.githubusercontent.com/dvyukov/6b340beab6483a036f4186e7378882ce/raw/cd1922185516453c201df8eded1d4b006a6d6a3a/gistfile1.txt
>
> I've queued up a fix for this:
>
> https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=9875fe3dc4b8cff1f1b440fb925054a5124403c3

I believe that this fix relies on call_rcu() having FIFO ordering; but
<https://www.kernel.org/doc/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html#Callback%20Registry>
says:

| call_rcu() normally acts only on CPU-local state[...] It simply
enqueues the rcu_head structure on a per-CPU list,

Is this fix really correct?