io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

Commit c9a40292a44e78f71258b8522655bffaf5753bdb upstream.

io_eventfd_do_signal() is invoked from an RCU callback, but when
dropping the reference to the io_ev_fd, it calls io_eventfd_free()
directly if the refcount drops to zero. This isn't correct, as any
potential freeing of the io_ev_fd should be deferred another RCU grace
period.

Just call io_eventfd_put() rather than open-code the dec-and-test and
free, which will correctly defer it another RCU grace period.

Fixes: 21a091b970cd ("io_uring: signal registered eventfd to process deferred task work")
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 9b58ba4616d40bc8c1f7c133fe3b8a52008dee36..480752fc3eb6e6490d99a5ab7ab72f04eccf4edc 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -479,6 +479,13 @@ static __cold void io_queue_deferred(struct io_ring_ctx *ctx)
        }
 }
 
+static void io_eventfd_free(struct rcu_head *rcu)
+{
+       struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
+
+       eventfd_ctx_put(ev_fd->cq_ev_fd);
+       kfree(ev_fd);
+}
 
 static void io_eventfd_ops(struct rcu_head *rcu)
 {
@@ -492,10 +499,8 @@ static void io_eventfd_ops(struct rcu_head *rcu)
         * ordering in a race but if references are 0 we know we have to free
         * it regardless.
         */
-       if (atomic_dec_and_test(&ev_fd->refs)) {
-               eventfd_ctx_put(ev_fd->cq_ev_fd);
-               kfree(ev_fd);
-       }
+       if (atomic_dec_and_test(&ev_fd->refs))
+               call_rcu(&ev_fd->rcu, io_eventfd_free);
 }
 
 static void io_eventfd_signal(struct io_ring_ctx *ctx)