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

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 <[email protected]>.

thanks,

greg k-h

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

From 447c19f3b5074409c794b350b10306e1da1ef4ba Mon Sep 17 00:00:00 2001
From: Pavel Begunkov <[email protected]>
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.

index 9ac5e278a91e..599102cc6dfc 100644
@@ -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) {