From: ebiederm@xmission.com (Eric W. Biederman)
To: Andy Lutomirski <luto@amacapital.net>
Cc: Linux Containers <containers@lists.linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Richard Weinberger <richard@nod.at>,
        Kenton Varda <kenton@sandstorm.io>, stable <stable@vger.kernel.org>
References: <52e0643bd47b1e5c65921d6e00aea1f724bb510a.1417281801.git.luto@amacapital.net>
	<87h9xez20g.fsf@x220.int.ebiederm.org>
	<CALCETrXOz4C7Tu8mggBtR=k47ZmkuAhinVUxWJSFyS1Ep0HvRw@mail.gmail.com>
	<87mw75ygwp.fsf@x220.int.ebiederm.org>
	<CALCETrVfO4sBdZcQiZXsofPZMj7pqKeVbX+4g3dAj6WjUca+1w@mail.gmail.com>
	<87fvcxyf28.fsf_-_@x220.int.ebiederm.org>
	<874mtdyexp.fsf_-_@x220.int.ebiederm.org>
	<CALCETrXyC7XPaqj6oe-TmyypOVc_CkZbF6UAAx8YfkyD=gEMOQ@mail.gmail.com>
	<87a935u3nj.fsf@x220.int.ebiederm.org>
	<CALCETrXkEOiyzpvqtXtk1f4sL+M1Q-Y6rV=K91ez3yv2nb4Y0Q@mail.gmail.com>
	<87388xodlj.fsf@x220.int.ebiederm.org>
	<CALCETrXWx2-ZejEHmOi7aSoF-qJMRGR5yseeMhuurZwJRrQbUg@mail.gmail.com>
	<87h9x5re41.fsf_-_@x220.int.ebiederm.org>
Date: Mon, 08 Dec 2014 16:07:02 -0600
In-Reply-To: <87h9x5re41.fsf_-_@x220.int.ebiederm.org> (Eric W. Biederman's
	message of "Mon, 08 Dec 2014 16:06:06 -0600")
Message-ID: <87bnndre2h.fsf_-_@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished
Sender: linux-kernel-owner@vger.kernel.org


setgroups is unique in not needing a valid mapping before it can be called,
in the case of setgroups(0, NULL) which drops all supplemental groups.

The design of the user namespace assumes that CAP_SETGID can not actually
be used until a gid mapping is established.  Therefore add a helper function
to see if the user namespace gid mapping has been established and call
that function in the setgroups permission check.

This is part of the fix for CVE-2014-8989, being able to drop groups
without privilege using user namespaces.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 include/linux/user_namespace.h | 9 +++++++++
 kernel/groups.c                | 7 ++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index e95372654f09..41cc26e5a350 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -37,6 +37,15 @@ struct user_namespace {
 
 extern struct user_namespace init_user_ns;
 
+static inline bool userns_gid_mappings_established(const struct user_namespace *ns)
+{
+	bool established;
+	smp_mb__before_atomic();
+	established = ACCESS_ONCE(ns->gid_map.nr_extents) != 0;
+	smp_mb__after_atomic();
+	return established;
+}
+
 #ifdef CONFIG_USER_NS
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
diff --git a/kernel/groups.c b/kernel/groups.c
index 02d8a251c476..e0335e44f76a 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -6,6 +6,7 @@
 #include <linux/slab.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
+#include <linux/user_namespace.h>
 #include <asm/uaccess.h>
 
 /* init to 2 - one for init_task, one to ensure it is never freed */
@@ -217,7 +218,11 @@ bool may_setgroups(void)
 {
 	struct user_namespace *user_ns = current_user_ns();
 
-	return ns_capable(user_ns, CAP_SETGID);
+	/* It is not safe to use setgroups until a gid mapping in
+	 * the user namespace has been established.
+	 */
+	return userns_gid_mappings_established(user_ns) &&
+		ns_capable(user_ns, CAP_SETGID);
 }
 
 /*
-- 
1.9.1

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