FAILED: patch "[PATCH] udmabuf: fix racy memfd sealing check" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-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]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 9cb189a882738c1d28b349d4e7c6a1ef9b3d8f87
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024122347-uncivil-decimal-1a11@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 9cb189a882738c1d28b349d4e7c6a1ef9b3d8f87 Mon Sep 17 00:00:00 2001
From: Jann Horn <[email protected]>
Date: Wed, 4 Dec 2024 17:26:19 +0100
Subject: [PATCH] udmabuf: fix racy memfd sealing check

The current check_memfd_seals() is racy: Since we first do check_memfd_seals() and then udmabuf_pin_folios() without holding any relevant lock across both, F_SEAL_WRITE can be set in between. This is problematic because we can end up holding pins to pages in a write-sealed memfd.

Fix it using the inode lock, that's probably the easiest way. In the future, we might want to consider moving this logic into memfd, especially if anyone else wants to use memfd_pin_folios().

index 8ce1f074c2d3..c1d8c2766d6d 100644
@@ -436,14 +436,19 @@ static long udmabuf_create(struct miscdevice *device,
 			goto err;
 		}
 
+		/*
+		 * Take the inode lock to protect against concurrent
+		 * memfd_add_seals(), which takes this lock in write mode.
+		 */
+		inode_lock_shared(file_inode(memfd));
 		ret = check_memfd_seals(memfd);
-		if (ret < 0) {
-			fput(memfd);
-			goto err;
-		}
+		if (ret)
+			goto out_unlock;
 
 		ret = udmabuf_pin_folios(ubuf, memfd, list[i].offset,
 					 list[i].size, folios);
+out_unlock:
+		inode_unlock_shared(file_inode(memfd));
 		fput(memfd);
 		if (ret)
 			goto err;