Re: [PATCH 2/2] fuse: wait for aborted connection before releasing last fuse_dev

[Bernd Schubert <bernd@bsbernd.com>]

On 5/18/26 11:06, Pavel Begunkov wrote:
> On 5/17/26 16:00, Bernd Schubert wrote:
>> On 5/17/26 14:59, Berkant Koc wrote:
>>> [You don't often get email from me@berkoc.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> From: Berkant Koc <me@berkoc.com>
>>>
>>> fuse_dev_release() on the last fuse_dev of a connection calls
>>> fuse_abort_conn(fc) and then immediately fuse_conn_put(fc). For io-uring
>>> backed connections fuse_abort_conn() reaches fuse_uring_abort(), which
>>> runs fuse_uring_teardown_all_queues() synchronously once and then
>>> schedules ring->async_teardown_work to run after
>>> FUSE_URING_TEARDOWN_INTERVAL (HZ/20). If the synchronous pass left
>>> queue_refs > 0 the work owns further accesses to ring->queues[*]->
>>> ent_avail_queue and ent_in_userspace entries.
>>>
>>> Meanwhile fuse_conn_put() can drop the last reference and arm
>>> delayed_release() via call_rcu(). After the RCU grace period
>>> delayed_release() calls fuse_uring_destruct(), which kfree()s the ring
>>> entries on each queue->ent_released list. The previously scheduled
>>> async_teardown_work then runs and walks per-queue lists that contain
>>> freed entries, producing a slab-use-after-free reported by KASAN at
>>> fuse_uring_teardown_all_queues+0xee reading ent->list.next from a
>>> freed kmalloc-192 region.
>>>
>>> fuse_wait_aborted() already exists for this purpose: it waits on
>>> fc->blocked_waitq for num_waiting to drain and then calls
>>> fuse_uring_wait_stopped_queues(), which waits for ring->queue_refs to
>>> reach zero. Call it between fuse_abort_conn() and fuse_conn_put() on
>>> the last-device path so the io-uring teardown work has fully drained
>>> before the connection can be torn down.
>>>
>>> Fixes: c090c8abae4b ("fuse: Add io-uring sqe commit and fetch support")
>>> Cc: stable@vger.kernel.org # 6.14+
>>> Tested-by: Berkant Koc <me@berkoc.com>
>>> Signed-off-by: Berkant Koc <me@berkoc.com>
>>> ---
>>>   fs/fuse/dev.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>>> index 5dda7080f4a9..7d9c06654a98 100644
>>> --- a/fs/fuse/dev.c
>>> +++ b/fs/fuse/dev.c
>>> @@ -2566,6 +2566,7 @@ int fuse_dev_release(struct inode *inode, struct file *file)
>>>                  if (last) {
>>>                          WARN_ON(fc->iq.fasync != NULL);
>>>                          fuse_abort_conn(fc);
>>> +                       fuse_wait_aborted(fc);
>>>                  }
>>>                  fuse_conn_put(fc);
>>>          }
>>
>> I might be wrong, but I don't think it is possible, Maybe Pavel or Jens
>> could help (added to CC). Basically as long as
>> fuse_uring_async_stop_queues() runs we do not have completed all
>> io-uring commands via io_uring_cmd_done() and as long as we do not have
>> completed these io-uring commands.
> 
> If I understand the question right, yes, fuse io_uring cmd requests hold
> a reference to the fuse file, so until you complete them the file will
> not get released.


Sorry, had totally messed up the phrase, can't read it myself.

What I mean is that the io-uring was set up with /dev/fuse as file and
as long as fuse holds non-completed 'struct io_uring_cmd *cmd' objects
there is a reference on the /dev/fuse fd, which blocks the call of
fuse_dev_release().


Thanks,
Bernd