linux-kernel - [PATCH] ovl: use I_MUTEX_PARENT when locking parent in ovl_create

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Message-id: <175430948898.2234665.11303643314523472166@noble.neil.brown.name>
Date: Mon, 04 Aug 2025 22:11:28 +1000
From: "NeilBrown" <neil@...wn.name>
To: "Amir Goldstein" <amir73il@...il.com>, "Al Viro" <viro@...iv.linux.org.uk>,
 "Miklos Szeredi"  <miklos@...redi.hu>,
 "Christian Brauner" <brauner@...nel.org>
Cc: Alan Huang <mmpgouride@...il.com>,
 syzbot <syzbot+7836a68852a10ec3d790@...kaller.appspotmail.com>,
 linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com, linux-unionfs@...r.kernel.org
Subject:
 [PATCH] ovl: use I_MUTEX_PARENT when locking parent in ovl_create_temp()


ovl_create_temp() treats "workdir" as a parent in which it creates an
object so it should use I_MUTEX_PARENT.

Prior to the commit identified below the lock was taken by the caller
which sometimes used I_MUTEX_PARENT and sometimes used I_MUTEX_NORMAL.
The use of I_MUTEX_NORMAL was incorrect but unfortunately copied into
ovl_create_temp().

Note to backporters: This patch only applies after the last Fixes given
below (post v6.16).  To fix the bug in v6.7 and later the
inode_lock() call in ovl_copy_up_workdir() needs to nest using
I_MUTEX_PARENT.

Link: https://lore.kernel.org/all/67a72070.050a0220.3d72c.0022.GAE@google.com/
Cc: stable@...r.kernel.org
Reported-by: syzbot+7836a68852a10ec3d790@...kaller.appspotmail.com
Fixes: c63e56a4a652 ("ovl: do not open/llseek lower file with upper sb_writers held")
Fixes: d2c995581c7c ("ovl: Call ovl_create_temp() without lock held.")
Signed-off-by: NeilBrown <neil@...wn.name>
---
 fs/overlayfs/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 70b8687dc45e..dbd63a74df4b 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -225,7 +225,7 @@ struct dentry *ovl_create_temp(struct ovl_fs *ofs, struct dentry *workdir,
 			       struct ovl_cattr *attr)
 {
 	struct dentry *ret;
-	inode_lock(workdir->d_inode);
+	inode_lock_nested(workdir->d_inode, I_MUTEX_PARENT);
 	ret = ovl_create_real(ofs, workdir,
 			      ovl_lookup_temp(ofs, workdir), attr);
 	inode_unlock(workdir->d_inode);