[Precise][CVE-2014-7970][PATCH 3/3] mnt: Prevent pivot_root from creating a loop in the mount tree

[Luis Henriques]

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

Andy Lutomirski recently demonstrated that when chroot is used to set
the root path below the path for the new ``root'' passed to pivot_root
the pivot_root system call succeeds and leaks mounts.

In examining the code I see that starting with a new root that is
below the current root in the mount tree will result in a loop in the
mount tree after the mounts are detached and then reattached to one
another.  Resulting in all kinds of ugliness including a leak of that
mounts involved in the leak of the mount loop.

Prevent this problem by ensuring that the new mount is reachable from
the current root of the mount tree.

[Added stable cc.  Fixes CVE-2014-7970.  --Andy]

Cc: stable at vger.kernel.org
Reported-by: Andy Lutomirski <luto at amacapital.net>
Reviewed-by: Andy Lutomirski <luto at amacapital.net>
Link: http://lkml.kernel.org/r/87bnpmihks.fsf@x220.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Andy Lutomirski <luto at amacapital.net>
(backported from commit 0d0826019e529f21c84687521d03f60cd241ca7d)
CVE-2014-7970
BugLink: http://bugs.launchpad.net/bugs/1383356
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/namespace.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index eaf076a2ed4a..5b10772f323a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2691,6 +2691,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 	/* make sure we can reach put_old from new_root */
 	if (!is_path_reachable(old.mnt, old.dentry, &new))
 		goto out4;
+	/* make certain new is below the root */
+	if (!is_path_reachable(new.mnt, new.dentry, &root))
+		goto out4;
 	br_write_lock(vfsmount_lock);
 	detach_mnt(new.mnt, &parent_path);
 	detach_mnt(root.mnt, &root_parent);
-- 
2.1.4