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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

-- 
Pavel Begunkov