[PATCH 5.12 088/161] io_uring: fix ltout double free on completion race

[Greg Kroah-Hartman]

From: Pavel Begunkov <asml.silence@xxxxxxxxx>

[ Upstream commit 447c19f3b5074409c794b350b10306e1da1ef4ba ]

Always remove linked timeout on io_link_timeout_fn() from the master
request link list, otherwise we may get use-after-free when first
io_link_timeout_fn() puts linked timeout in the fail path, and then
will be found and put on master's free.

Cc: stable@xxxxxxxxxxxxxxx # 5.10+
Fixes: 90cd7e424969d ("io_uring: track link timeout's master explicitly")
Reported-and-tested-by: syzbot+5a864149dd970b546223@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
Link: https://lore.kernel.org/r/69c46bf6ce37fec4fdcd98f0882e18eb07ce693a.1620990121.git.asml.silence@xxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/io_uring.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index dd8b3fac877c..359d1abb089c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6289,10 +6289,11 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	 * We don't expect the list to be empty, that will only happen if we
 	 * race with the completion of the linked work.
 	 */
-	if (prev && req_ref_inc_not_zero(prev))
+	if (prev) {
 		io_remove_next_linked(prev);
-	else
-		prev = NULL;
+		if (!req_ref_inc_not_zero(prev))
+			prev = NULL;
+	}
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
 	if (prev) {
-- 
2.30.2