From c782fc8ada811569811c0fdea5c3412511be95d0 Mon Sep 17 00:00:00 2001 From: Chris Mason Date: Fri, 16 Oct 2026 21:17:21 +0000 Subject: [PATCH] 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 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 Tested-by: Hungyu Lin Signed-off-by: Amir Goldstein --- 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 d9174eaccce2..8bc970748b96 100644 --- a/fs/overlayfs/super.c +++ b/fs/overlayfs/super.c @@ -1387,7 +1387,6 @@ 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); @@ -1571,6 +1570,17 @@ static int ovl_fill_super_creds(struct fs_context *fc, struct super_block *sb) if (!sb->s_root) 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(sb->s_root)) != ofs->casefold) { + pr_err("casefold flag of the root directory has changed\n"); + dput(sb->s_root); + sb->s_root = NULL; + return -EINVAL; + } + return 0; out_free_oe: