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From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Wed, 8 Oct 2014 10:42:27 -0700
Subject: [PATCH] mnt: Prevent pivot_root from creating a loop in the mount
 tree

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.

Upstream-status: Submitted for 3.18
Bugzilla: 1151095,1151484

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/namespace.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 348562f14e93..c8e3034ff4b2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2913,6 +2913,9 @@ 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;
 	root_mp->m_count++; /* pin it so it won't go away */
 	lock_mount_hash();
 	detach_mnt(new_mnt, &parent_path);
-- 
1.9.3