ovl: fix decode of dir file handle with multi lower layers

When decoding a lower file handle, we first call ovl_check_origin_fh()
with connected=false to get any real lower dentry for overlay inode
cache lookup.

If the real dentry is a disconnected dir dentry, ovl_check_origin_fh()
is called again with connected=true to get a connected real dentry
and find the lower layer the real dentry belongs to.

If the first call returned a connected real dentry, we use it to
lookup an overlay connected dentry, but the first ovl_check_origin_fh()
call with connected=false did not check that the found dentry is under
the root of the layer (see ovl_acceptable()), it only checked that
the found dentry super block matches the uuid of the lower file handle.

In case there are multiple lower layers on the same fs and the found
dentry is not from the top most lower layer, using the layer index
returned from the first ovl_check_origin_fh() is wrong and we end
up failing to decode the file handle.

Fix this by always calling ovl_check_origin_fh() with connected=true
if we got a directory dentry in the first call.

Fixes: 8b58924ad55c ("ovl: lookup in inode cache first when decoding...")
Cc: <stable@vger.kernel.org> # v4.17
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/export.c

@@ -754,9 +754,8 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 		goto out;
 	}
 
-	/* Otherwise, get a connected non-upper dir or disconnected non-dir */
-	if (d_is_dir(origin.dentry) &&
-	    (origin.dentry->d_flags & DCACHE_DISCONNECTED)) {
+	/* Find origin.dentry again with ovl_acceptable() layer check */
+	if (d_is_dir(origin.dentry)) {
 		dput(origin.dentry);
 		origin.dentry = NULL;
 		err = ovl_check_origin_fh(ofs, fh, true, NULL, &stack);
@@ -769,6 +768,7 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 			goto out_err;
 	}
 
+	/* Get a connected non-upper dir or disconnected non-dir */
 	dentry = ovl_get_dentry(sb, NULL, &origin, index);
 
 out: