[patch added to the 3.12 stable tree] mnt: Fix fs_fully_visible to verify the root directory is visible

[Jiri Slaby <jslaby@suse.cz>]

From: "Eric W. Biederman" <ebiederm@xmission.com>

This patch has been added to the 3.12 stable tree. If you have any
objections, please let us know.

===============

commit 7e96c1b0e0f495c5a7450dc4aa7c9a24ba4305bd upstream.

This fixes a dumb bug in fs_fully_visible that allows proc or sys to
be mounted if there is a bind mount of part of /proc/ or /sys/ visible.

Reported-by: Eric Windisch <ewindisch@docker.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 fs/namespace.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7c3c0f6d2744..247f34d43dda 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2940,6 +2940,12 @@ bool fs_fully_visible(struct file_system_type *type)
 		if (mnt->mnt.mnt_sb->s_type != type)
 			continue;
 
+		/* This mount is not fully visible if it's root directory
+		 * is not the root directory of the filesystem.
+		 */
+		if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
+			continue;
+
 		/* This mount is not fully visible if there are any child mounts
 		 * that cover anything except for empty directories.
 		 */
-- 
2.3.7