[PATCH 6.18.y v2] ovl: fail the mount if the casefold flag has changed

[PATCH 6.18.y v2] ovl: fail the mount if the casefold flag has changed

[Hungyu Lin]

From: Chris Mason <mason@kernel.org>

[ Upstream commit c782fc8ada811569811c0fdea5c3412511be95d0 ]

An unprivileged user in a user namespace can repeatedly trigger a kernel
warning with their own overlay mounts. They do so by changing the
casefold flag of the topmost layer's empty root, the top directory of
that layer, after the layers are set and before the mount is created.

The casefold flag of a directory makes name lookups in that directory
case-insensitive. All layers of an overlay must agree on casefolding.

ovl_mount_dir_check() reads the casefold flag of a layer root when
fsconfig() sets that layer, and saves the result in ofs->casefold. The
mount is created later, by another fsconfig() call with
FSCONFIG_CMD_CREATE.

The casefold flag of an empty directory can be changed.

The casefold flag of the overlay root must match ofs->casefold.
ovl_get_root() copies the flag from the topmost layer root, as it is
when the mount is created. On a mismatch, ovl_get_root() only does a
WARN_ON(), and the mount continues. The check is not a WARN_ON_ONCE(),
so each such mount warns again.

An unprivileged user needs CONFIG_UNICODE, and a tmpfs mounted with the
casefold option in their namespace.

Reproduced by changing the casefold flag of the empty upper directory
after the layers were set and before FSCONFIG_CMD_CREATE, with tmpfs and
ext4 layers, and as uid 1000 in a user namespace.

Fix this by replacing the WARN_ON() with a check that fails the mount
with -EINVAL.

Fixes: f9377faaeae0 ("ovl: Add S_CASEFOLD as part of the inode flag to be copied")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Tested-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[backport: In 6.18, ovl_fill_super() handles credential restoration and
filesystem cleanup. Drop the unpublished root dentry and go to out_err,
skipping out_free_oe because the root inode already owns oe.]
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
Changes in v2:
- Restore the upstream commit message and move the 6.18 adjustment
  into a bracketed note above my sign-off, as Amir suggested.

OpenAI Codex assisted with adapting the backport and running the QEMU
checks.

 fs/overlayfs/super.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 8d4ef5b..52d6f32 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1370,7 +1370,6 @@ static struct dentry *ovl_get_root(struct super_block *sb,
 	ovl_dentry_set_flag(OVL_E_CONNECTED, root);
 	ovl_set_upperdata(d_inode(root));
 	ovl_inode_init(d_inode(root), &oip, ino, fsid);
-	WARN_ON(!!IS_CASEFOLDED(d_inode(root)) != ofs->casefold);
 	ovl_dentry_init_flags(root, upperdentry, oe, DCACHE_OP_WEAK_REVALIDATE);
 	/* root keeps a reference of upperdentry */
 	dget(upperdentry);
@@ -1576,6 +1575,17 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 	if (!root_dentry)
 		goto out_free_oe;
 
+	/*
+	 * The casefold flag of an empty layer root may have changed since the
+	 * mount options were parsed. The root inode owns oe by now.
+	 */
+	if (!!IS_CASEFOLDED(d_inode(root_dentry)) != ofs->casefold) {
+		pr_err("casefold flag of the root directory has changed\n");
+		dput(root_dentry);
+		err = -EINVAL;
+		goto out_err;
+	}
+
 	sb->s_root = root_dentry;
 
 	ovl_revert_creds(old_cred);
-- 
2.50.1 (Apple Git-155)

Re: [PATCH 6.18.y v2] ovl: fail the mount if the casefold flag has changed

[Sasha Levin]

On 2026-11-12, Hungyu Lin wrote:
> [PATCH 6.18.y v2] ovl: fail the mount if the casefold flag has changed
> An unprivileged user in a user namespace can repeatedly trigger a kernel
> warning with their own overlay mounts. They do so by changing the
> casefold flag of the topmost layer's empty root, the top directory of

Queued for 6.18, thanks.


-- 
Thanks,
Sasha