userns: Check euid no fsuid when establishing an unprivileged uid mapping

commit 80dd00a23784b384ccea049bfb3f259d3f973b9d upstream.

setresuid allows the euid to be set to any of uid, euid, suid, and
fsuid.  Therefor it is safe to allow an unprivileged user to map
their euid and use CAP_SETUID privileged with exactly that uid,
as no new credentials can be obtained.

I can not find a combination of existing system calls that allows setting
uid, euid, suid, and fsuid from the fsuid making the previous use
of fsuid for allowing unprivileged mappings a bug.

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

Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a5809c42a1b31ffebb30b557aae3db50cad106f4..6e495bd672a73078f7442488482c53d469b61783 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -805,7 +805,7 @@ static bool new_idmap_permitted(const struct file *file,
                u32 id = new_map->extent[0].lower_first;
                if (cap_setid == CAP_SETUID) {
                        kuid_t uid = make_kuid(ns->parent, id);
-                       if (uid_eq(uid, file->f_cred->fsuid))
+                       if (uid_eq(uid, file->f_cred->euid))
                                return true;
                }
        }