[PATCH 22/34] union-mount: Prevent improper union-related remounts

[Valerie Aurora <vaurora@redhat.com>]

A remount request must (a) not convert a union to a non-union (or vice
versa), or (b) make the topmost layer of a union read-only.

Note that we only have to worry about attempts to remount the vfsmount
of the topmost read-write of the union (the one with MNT_UNION set).
The vfsmounts of the read-only layers are hidden in a cloned tree
hanging of the superblock of the topmost layer and aren't visible to
userspace.

Signed-off-by: Valerie Aurora <vaurora@redhat.com>
---
 fs/namespace.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ff83cee..61256e6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1824,6 +1824,18 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 	if (!check_mnt(path->mnt))
 		return -EINVAL;
 
+	if ((path->mnt->mnt_flags & MNT_UNION) &&
+	    !(mnt_flags & MNT_UNION))
+		return -EINVAL;
+
+	if ((mnt_flags & MNT_UNION) &&
+	    !(path->mnt->mnt_flags & MNT_UNION))
+		return -EINVAL;
+
+	if ((path->mnt->mnt_flags & MNT_UNION) &&
+	    (mnt_flags & MNT_READONLY))
+		return -EINVAL;
+
 	if (path->dentry != path->mnt->mnt_root)
 		return -EINVAL;
 
-- 
1.6.3.3