[PATCH 6.6.y] ovl: inherit FMODE_NONOTIFY when opening directory realfiles

[Sasha Levin <sashal@kernel.org>]

From: Amir Goldstein <amir73il@gmail.com>

[ Upstream commit 323899b4ed233d5b6e2fc160c16fbf39cf0317da ]

When fanotify hands a listener an fd for a permission event on an
overlay directory, ovl_dir_open_realfile() opens the real directory via
dentry_open(), not backing_file_open(), so the lower open could still
raise FS_OPEN_PERM and deadlock a listener that also marks the backing
filesystem.

Export dentry_open_nonotify() and call it from ovl_path_open_realfile()
when the overlay user file must not generate events.  Ordinary
ovl_path_open() callers are unchanged.

Reported-by: Christian Brauner (Amutable) <brauner@kernel.org>
Closes: https://lore.kernel.org/all/20261030-welpen-anbinden-nullen-b2432cee1d10@brauner/
Fixes: 130fdbc3d1f99 ("ovl: pass correct flags for opening real directory")
Cc: stable@vger.kernel.org
Tested-by: Edvard Conrad <edvard@tuta.io>
Link: https://lore.kernel.org/ltp/P5Iny41--F-9@tuta.io/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[ sashal: Replaced dentry_open_nonotify() with __FMODE_NONOTIFY flags passed through the existing ovl_path_open() helper. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/overlayfs/readdir.c | 10 +++++++++-
 fs/overlayfs/util.c    |  2 +-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 9252e20bdab8b..ed79c2ad3593e 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -855,9 +855,17 @@ static struct file *ovl_dir_open_realfile(const struct file *file,
 {
 	struct file *res;
 	const struct cred *old_cred;
+	int flags = O_RDONLY | (file->f_flags & O_LARGEFILE);
+
+	/*
+	 * Inherit FMODE_NONOTIFY from a fanotify permission fd so a listener
+	 * that also watches the real filesystem does not deadlock.
+	 */
+	if (file->f_mode & FMODE_NONOTIFY)
+		flags |= __FMODE_NONOTIFY;
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
-	res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
+	res = ovl_path_open(realpath, flags);
 	revert_creds(old_cred);
 
 	return res;
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 0da1cd01d01cf..0a40a4950e9f6 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -586,7 +586,7 @@ struct file *ovl_path_open(const struct path *path, int flags)
 	struct mnt_idmap *real_idmap = mnt_idmap(path->mnt);
 	int err, acc_mode;
 
-	if (flags & ~(O_ACCMODE | O_LARGEFILE))
+	if (flags & ~(O_ACCMODE | O_LARGEFILE | __FMODE_NONOTIFY))
 		BUG();
 
 	switch (flags & O_ACCMODE) {
-- 
2.53.0