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

[Jens Axboe]

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@xxxxxxxxx> wrote:
>>> On 3/6/20 7:57 AM, Jann Horn wrote:
>>>> +paulmck
>>>>
>>>> On Wed, Mar 4, 2020 at 3:40 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>>> On 3/4/20 12:59 AM, Dmitry Vyukov wrote:
>>>>>> On Fri, Feb 7, 2020 at 9:14 AM syzbot
>>>>>> <syzbot+e017e49c39ab484ac87a@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>>
>>>>>> +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;

-- 
Jens Axboe