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

[Jens Axboe <axboe@kernel.dk>]

On 3/6/20 10:19 AM, Paul E. McKenney wrote:
> On Fri, Mar 06, 2020 at 10:00:19AM -0700, Jens Axboe wrote:
>> On 3/6/20 9:44 AM, Paul E. McKenney wrote:
>>> On Fri, Mar 06, 2020 at 04:36:20PM +0100, Jann Horn wrote:
>>>> On Fri, Mar 6, 2020 at 4:34 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>> On 3/6/20 7:57 AM, Jann Horn wrote:
>>>>>> +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,
>>>
>>> Indeed.  For but one example, if there was a CPU-to-CPU migration between
>>> the two call_rcu() invocations, it would not be at all surprising for
>>> the two callbacks to execute out of order.
>>>
>>>>>> Is this fix really correct?
>>>>>
>>>>> That's a good point, there's a potentially stronger guarantee we need
>>>>> here that isn't "nobody is inside an RCU critical section", but rather
>>>>> that we're depending on a previous call_rcu() to have happened. Hence I
>>>>> think you are right - it'll shrink the window drastically, since the
>>>>> previous callback is already queued up, but it's not a full close.
>>>>>
>>>>> Hmm...
>>>>
>>>> You could potentially hack up the semantics you want by doing a
>>>> call_rcu() whose callback does another call_rcu(), or something like
>>>> that - but I'd like to hear paulmck's opinion on this first.
>>>
>>> That would work!
>>>
>>> Or, alternatively, do an rcu_barrier() between the two calls to
>>> call_rcu(), assuming that the use case can tolerate rcu_barrier()
>>> overhead and latency.
>>
>> If the nested call_rcu() works, that seems greatly preferable to needing
>> the rcu_barrier(), even if that would not be a showstopper for me. The
>> nested call_rcu() is just a bit odd, but with a comment it should be OK.
>>
>> Incremental here I'm going to test, would just fold in of course.
>>
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index f3218fc81943..95ba95b4d8ec 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -5330,7 +5330,7 @@ static void io_file_ref_kill(struct percpu_ref *ref)
>>  	complete(&data->done);
>>  }
>>  
>> -static void io_file_ref_exit_and_free(struct rcu_head *rcu)
>> +static void __io_file_ref_exit_and_free(struct rcu_head *rcu)
>>  {
>>  	struct fixed_file_data *data = container_of(rcu, struct fixed_file_data,
>>  							rcu);
>> @@ -5338,6 +5338,18 @@ static void io_file_ref_exit_and_free(struct rcu_head *rcu)
>>  	kfree(data);
>>  }
>>  
>> +static void io_file_ref_exit_and_free(struct rcu_head *rcu)
>> +{
>> +	/*
>> +	 * We need to order our exit+free call again the potentially
>> +	 * existing call_rcu() for switching to atomic. One way to do that
>> +	 * is to have this rcu callback queue the final put and free, as we
>> +	 * could otherwise a pre-existing atomic switch complete _after_
>> +	 * the free callback we queued.
>> +	 */
>> +	call_rcu(rcu, __io_file_ref_exit_and_free);
>> +}
>> +
>>  static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
>>  {
>>  	struct fixed_file_data *data = ctx->file_data;
> 
> Looks good to me!

Thanks Paul! I talked to Paul in private, but here's the final version
with updated comment and attributions.

https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=c1e2148f8ecb26863b899d402a823dab8e26efd1

-- 
Jens Axboe