CVE-2018-18955: Incorrect Authorization (CWE-863)
value: In the Linux kernel 4.15.x through 4.19.x before 4.19.2, map_write() in kernel/user_namespace.c allows privilege escalation because it mishandles nested user namespaces with more than 5 UID or GID ranges. A user who has CAP_SYS_ADMIN in an affected user namespace can bypass access controls on resources outside the namespace, as demonstrated by reading /etc/shadow. This occurs because an ID transformation takes place properly for the namespaced-to-kernel direction but not for the kernel-to-namespaced direction.
CWE description: The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.
CVSS v3 base score: 7
Published: 2018-11-16T20:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: kernel/user_namespace.c
Commit date: 2018-11-07 23:51:16 -0600
Commit message: 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>
Tested-by: Eric W. Biederman <ebiederm@xmission.com>
Reviewed-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Diff: 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
@@ -972,14 +972,10 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	ret = -EPERM;
 	/* Validate the user is allowed to use user id's mapped to. */
 	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.
 	 */
 	for (idx = 0; idx < new_map.nr_extents; idx++) {
@@ -1002,10 +998,18 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 			goto out;
 
 		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,
 		       new_map.nr_extents * sizeof(new_map.extent[0]));
 	} else {

Vulnerable code: 	ret = sort_idmaps(&new_map);
	if (ret < 0)
		goto out;

Fixed code: 	/*
	 * 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;