FAILED: patch "[PATCH] io_uring: fix ltout double free on completion race" failed to apply to 5.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 447c19f3b5074409c794b350b10306e1da1ef4ba Mon Sep 17 00:00:00 2001
From: Pavel Begunkov <asml.silence@gmail.com>
Date: Fri, 14 May 2021 12:02:50 +0100
Subject: [PATCH] io_uring: fix ltout double free on completion race

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@vger.kernel.org # 5.10+
Fixes: 90cd7e424969d ("io_uring: track link timeout's master explicitly")
Reported-and-tested-by: syzbot+5a864149dd970b546223@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/69c46bf6ce37fec4fdcd98f0882e18eb07ce693a.1620990121.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9ac5e278a91e..599102cc6dfc 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6354,10 +6354,11 @@ 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) {