FAILED: patch "[PATCH] btrfs: fix use-after-free when COWing tree bock and tracing" 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 44f52bbe96dfdbe4aca3818a2534520082a07040
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024123021-campus-undoing-7345@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 44f52bbe96dfdbe4aca3818a2534520082a07040 Mon Sep 17 00:00:00 2001
From: Filipe Manana <[email protected]>
Date: Wed, 11 Dec 2024 16:08:07 +0000
Subject: [PATCH] btrfs: fix use-after-free when COWing tree bock and tracing
is enabled

When a COWing a tree block, at btrfs_cow_block(), and we have the tracepoint trace_btrfs_cow_block() enabled and preemption is also enabled (CONFIG_PREEMPT=y), we can trigger a use-after-free in the COWed extent buffer while inside the tracepoint code. This is because in some paths that call btrfs_cow_block(), such as btrfs_search_slot(), we are holding the last reference on the extent buffer @buf so btrfs_force_cow_block() drops the last reference on the @buf extent buffer when it calls free_extent_buffer_stale(buf), which schedules the release of the extent buffer with RCU. This means that if we are on a kernel with preemption, the current task may be preempted before calling trace_btrfs_cow_block() and the extent buffer already released by the time trace_btrfs_cow_block() is called, resulting in a use-after-free.

Fix this by moving the trace_btrfs_cow_block() from btrfs_cow_block() to btrfs_force_cow_block() before the COWed extent buffer is freed. This also has a side effect of invoking the tracepoint in the tree defrag code, at defrag.c:btrfs_realloc_node(), since btrfs_force_cow_block() is called there, but this is fine and it was actually missing there.

index 693dc27ffb89..185985a337b3 100644
@@ -654,6 +654,8 @@ int btrfs_force_cow_block(struct btrfs_trans_handle *trans,
 			goto error_unlock_cow;
 		}
 	}
+
+	trace_btrfs_cow_block(root, buf, cow);
 	if (unlock_orig)
 		btrfs_tree_unlock(buf);
 	free_extent_buffer_stale(buf);

@@ -710,7 +712,6 @@ int btrfs_cow_block(struct btrfs_trans_handle *trans,
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	u64 search_start;
-	int ret;
 
 	if (unlikely(test_bit(BTRFS_ROOT_DELETING, &root->state))) {
 		btrfs_abort_transaction(trans, -EUCLEAN);

@@ -751,12 +752,8 @@ int btrfs_cow_block(struct btrfs_trans_handle *trans,
 	 * Also We don't care about the error, as it's handled internally.
 	 */
 	btrfs_qgroup_trace_subtree_after_cow(trans, root, buf);
-	ret = btrfs_force_cow_block(trans, root, buf, parent, parent_slot,
-				    cow_ret, search_start, 0, nest);
-
-	trace_btrfs_cow_block(root, buf, *cow_ret);
-
-	return ret;
+	return btrfs_force_cow_block(trans, root, buf, parent, parent_slot,
+				     cow_ret, search_start, 0, nest);
 }
 ALLOW_ERROR_INJECTION(btrfs_cow_block, ERRNO);