[PATCH v6 1/2] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[Mark Salyzyn <salyzyn@android.com>]

Assumption never checked, should fail if the mounter creds are not
sufficient.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com

v5:
- dependency of "overlayfs: override_creds=off option bypass creator_cred"

v6:
- rebase
---
 fs/overlayfs/namei.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index efd372312ef1..aa012b6bd46e 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -163,6 +163,11 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
 	if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
 		return NULL;
 
+	if (!capable(CAP_DAC_READ_SEARCH)) {
+		origin = ERR_PTR(-EPERM);
+		goto out;
+	}
+
 	bytes = (fh->len - offsetof(struct ovl_fh, fid));
 	real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
 				  bytes >> 2, (int)fh->type,
-- 
2.19.1.930.g4563a0d9d0-goog