[CFT][PATCH 3/8] userns: Don't allow unprivileged creation of gid mappings

From: Eric W. Biederman
Date: Tue Dec 09 2014 - 15:41:50 EST

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@xxxxxxxxxxxxxxx
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
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;


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