[PATCH 5.15 078/141] io_uring: fix mutex_unlock with unreferenced ctx

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5.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Pavel Begunkov <asml.silence@xxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Fixes: 04fc6c802d ("io_uring: save ctx put/get for task_work submit")
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
Link: https://lore.kernel.org/io-uring/CAG48ez3xSoYb+45f1RLtktROJrpiDQ1otNvdR+YLQf7m+Krj5Q@xxxxxxxxxxxxxx/
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 io_uring/io_uring.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2601,6 +2601,7 @@ static void io_iopoll_try_reap_events(st
 	if (!(ctx->flags & IORING_SETUP_IOPOLL))
 		return;
 
+	percpu_ref_get(&ctx->refs);
 	mutex_lock(&ctx->uring_lock);
 	while (!list_empty(&ctx->iopoll_list)) {
 		unsigned int nr_events = 0;
@@ -2622,6 +2623,7 @@ static void io_iopoll_try_reap_events(st
 		}
 	}
 	mutex_unlock(&ctx->uring_lock);
+	percpu_ref_put(&ctx->refs);
 }
 
 static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
@@ -9771,12 +9773,7 @@ static void io_ring_exit_work(struct wor
 	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));






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