ovl: Always reject mounting over case-insensitive directories

overlayfs relies on the filesystem setting DCACHE_OP_HASH or
DCACHE_OP_COMPARE to reject mounting over case-insensitive directories.

Since commit bb9cd91 ("fscrypt: Have filesystems handle their
d_ops"), we set ->d_op through a hook in ->d_lookup, which
means the root dentry won't have them, causing the mount to accidentally
succeed.

In v6.7-rc7, the following sequence will succeed to mount, but any
dentry other than the root dentry will be a "weird" dentry to ovl and
fail with EREMOTE.

  mkfs.ext4 -O casefold lower.img
  mount -O loop lower.img lower
  mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=work ovl /mnt

Mounting on a subdirectory fails, as expected, because DCACHE_OP_HASH
and DCACHE_OP_COMPARE are properly set by ->lookup.

Fix by explicitly rejecting superblocks that allow case-insensitive
dentries. Yes, this will be solved when we move d_op configuration back
to ->s_d_op. Yet, we better have an explicit fix to avoid messing up
again.

While there, re-sort the entries to have more descriptive error messages
first.

Fixes: bb9cd91 ("fscrypt: Have filesystems handle their d_ops")
Acked-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20240221171412.10710-2-krisman@suse.de
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>

Author: krisman

fs/overlayfs/params.c

@@ -280,12 +280,20 @@ static int ovl_mount_dir_check(struct fs_context *fc, const struct path *path,
 {
 	struct ovl_fs_context *ctx = fc->fs_private;
 
-	if (ovl_dentry_weird(path->dentry))
-		return invalfc(fc, "filesystem on %s not supported", name);
-
 	if (!d_is_dir(path->dentry))
 		return invalfc(fc, "%s is not a directory", name);
 
+	/*
+	 * Root dentries of case-insensitive capable filesystems might
+	 * not have the dentry operations set, but still be incompatible
+	 * with overlayfs.  Check explicitly to prevent post-mount
+	 * failures.
+	 */
+	if (sb_has_encoding(path->mnt->mnt_sb))
+		return invalfc(fc, "case-insensitive capable filesystem on %s not supported", name);
+
+	if (ovl_dentry_weird(path->dentry))
+		return invalfc(fc, "filesystem on %s not supported", name);
 
 	/*
 	 * Check whether upper path is read-only here to report failures

include/linux/fs.h

@@ -3282,6 +3282,15 @@ extern int generic_check_addressable(unsigned, u64);
 
 extern void generic_set_encrypted_ci_d_ops(struct dentry *dentry);
 
+static inline bool sb_has_encoding(const struct super_block *sb)
+{
+#if IS_ENABLED(CONFIG_UNICODE)
+	return !!sb->s_encoding;
+#else
+	return false;
+#endif
+}
+
 int may_setattr(struct mnt_idmap *idmap, struct inode *inode,
 		unsigned int ia_valid);
 int setattr_prepare(struct mnt_idmap *, struct dentry *, struct iattr *);