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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 kernel/user_namespace.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 27c8dab48c07..1ce6d67c07b7 100644
--- 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;
 		}
 	}
 

-- 
1.9.1