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. [Added stable cc. Fixes CVE-2014-7970. --Andy] Cc: [email protected] Reported-by: Andy Lutomirski <[email protected]> Reviewed-by: Andy Lutomirski <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: "Eric W. Biederman" <[email protected]> Signed-off-by: Andy Lutomirski <[email protected]>
CamonZ · Oct 14, 2014 · 0d08260 · 0d08260
1 parent c798360
commit 0d08260
Showing 1 changed file with 3 additions and 0 deletions.
diff --git a/fs/namespace.c b/fs/namespace.c
@@ -2820,6 +2820,9 @@ 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);