io_uring: fix mutex_unlock with unreferenced ctx

commit f7b32e785042d2357c5abc23ca6db1b92c91a070 upstream.

Callers of mutex_unlock() have to make sure that the mutex stays alive
for the whole duration of the function call. For io_uring that means
that the following pattern is not valid unless we ensure that the
context outlives the mutex_unlock() call.

mutex_lock(&ctx->uring_lock);
req_put(req); // typically via io_req_task_submit()
mutex_unlock(&ctx->uring_lock);

Most contexts are fine: io-wq pins requests, syscalls hold the file,
task works are taking ctx references and so on. However, the task work
fallback path doesn't follow the rule.

Cc:  <stable@vger.kernel.org>
Fixes: 04fc6c802d ("io_uring: save ctx put/get for task_work submit")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/io-uring/CAG48ez3xSoYb+45f1RLtktROJrpiDQ1otNvdR+YLQf7m+Krj5Q@mail.gmail.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index f413ebed81ab..35894955b454 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1377,6 +1377,7 @@ static __cold void io_iopoll_try_reap_events(struct io_ring_ctx *ctx)
 	if (!(ctx->flags & IORING_SETUP_IOPOLL))
 		return;
 
+	percpu_ref_get(&ctx->refs);
 	mutex_lock(&ctx->uring_lock);
 	while (!wq_list_empty(&ctx->iopoll_list)) {
 		/* let it sleep and repeat later if can't complete a request */
@@ -1394,6 +1395,7 @@ static __cold void io_iopoll_try_reap_events(struct io_ring_ctx *ctx)
 		}
 	}
 	mutex_unlock(&ctx->uring_lock);
+	percpu_ref_put(&ctx->refs);
 }
 
 static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
@@ -2800,12 +2802,7 @@ static __cold void io_ring_exit_work(struct work_struct *work)
 	init_completion(&exit.completion);
 	init_task_work(&exit.task_work, io_tctx_exit_cb);
 	exit.ctx = ctx;
-	/*
-	 * Some may use context even when all refs and requests have been put,
-	 * and they are free to do so while still holding uring_lock or
-	 * completion_lock, see io_req_task_submit(). Apart from other work,
-	 * this lock/unlock section also waits them to finish.
-	 */
+
 	mutex_lock(&ctx->uring_lock);
 	while (!list_empty(&ctx->tctx_list)) {
 		WARN_ON_ONCE(time_after(jiffies, timeout));