[PATCH 1/1][B/C/D] userns: also map extents in the reverse map to kernel IDs

[Tyler Hicks]

From: Jann Horn <jannh at google.com>

BugLink: https://launchpad.net/bugs/1801924

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 at vger.kernel.org
Signed-off-by: Jann Horn <jannh at google.com>
Tested-by: Eric W. Biederman <ebiederm at xmission.com>
Reviewed-by: Eric W. Biederman <ebiederm at xmission.com>
Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>

CVE-2018-18955

(cherry picked from commit d2f007dbe7e4c9583eea6eb04d60001e85c6f1bd)
Signed-off-by: Tyler Hicks <tyhicks at canonical.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 840ab1fbbc3b..80ed67b1acd2 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -980,10 +980,6 @@ 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.
@@ -1010,6 +1006,14 @@ 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,
-- 
2.7.4