userns: Don't allow unprivileged creation of gid mappings

As any gid mapping will allow and must allow for backwards
compatibility dropping groups don't allow any gid mappings to be
established without CAP_SETGID in the parent user namespace.

For a small class of applications this change breaks userspace
and removes useful functionality.  This small class of applications
includes tools/testing/selftests/mount/unprivilged-remount-test.c

Most of the removed functionality will be added back with the addition
of a one way knob to disable setgroups.  Once setgroups is disabled
setting the gid_map becomes as safe as setting the uid_map.

For more common applications that set the uid_map and the gid_map
with privilege this change will have no affect.

This is part of a fix for CVE-2014-8989.

Cc: stable@vger.kernel.org
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 27c8dab48c076b624109a06a7d7be154c594ae04..1ce6d67c07b742fc5517a6f1ebce3ce74cdfb5c8 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -821,10 +821,6 @@ static bool new_idmap_permitted(const struct file *file,
                        kuid_t uid = make_kuid(ns->parent, id);
                        if (uid_eq(uid, file->f_cred->fsuid))
                                return true;
-               } else if (cap_setid == CAP_SETGID) {
-                       kgid_t gid = make_kgid(ns->parent, id);
-                       if (gid_eq(gid, file->f_cred->fsgid))
-                               return true;
                }
        }