[3.13.y-ckt stable] Patch "userns: Document what the invariant required for safe unprivileged mappings." has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    userns: Document what the invariant required for safe unprivileged mappings.

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt14.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From be9edd8bb52362735dc6b4418106c2b596b1b45d Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm at xmission.com>
Date: Fri, 5 Dec 2014 17:51:47 -0600
Subject: userns: Document what the invariant required for safe unprivileged
 mappings.

commit 0542f17bf2c1f2430d368f44c8fcf2f82ec9e53e upstream.

The rule is simple.  Don't allow anything that wouldn't be allowed
without unprivileged mappings.

It was previously overlooked that establishing gid mappings would
allow dropping groups and potentially gaining permission to files and
directories that had lesser permissions for a specific group than for
all other users.

This is the rule needed to fix CVE-2014-8989 and prevent any other
security issues with new_idmap_permitted.

The reason for this rule is that the unix permission model is old and
there are programs out there somewhere that take advantage of every
little corner of it.  So allowing a uid or gid mapping to be
established without privielge that would allow anything that would not
be allowed without that mapping will result in expectations from some
code somewhere being violated.  Violated expectations about the
behavior of the OS is a long way to say a security issue.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/user_namespace.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 8bdb421..1ae8d2c 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -804,7 +804,9 @@ static bool new_idmap_permitted(const struct file *file,
 				struct user_namespace *ns, int cap_setid,
 				struct uid_gid_map *new_map)
 {
-	/* Allow mapping to your own filesystem ids */
+	/* Don't allow mappings that would allow anything that wouldn't
+	 * be allowed without the establishment of unprivileged mappings.
+	 */
 	if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1)) {
 		u32 id = new_map->extent[0].lower_first;
 		if (cap_setid == CAP_SETUID) {
--
1.9.1