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>
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	<87a935u3nj.fsf@x220.int.ebiederm.org>
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	<87388xodlj.fsf@x220.int.ebiederm.org>
	<CALCETrXWx2-ZejEHmOi7aSoF-qJMRGR5yseeMhuurZwJRrQbUg@mail.gmail.com>
	<87h9x5re41.fsf_-_@x220.int.ebiederm.org>
	<87mw6xpzb0.fsf_-_@x220.int.ebiederm.org>
	<CALCETrU-o5mPr1jCaLXDuuF6J2N470zAtx=8Fa-SjF=ZpdE8mQ@mail.gmail.com>
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	<87a92xn2io.fsf@x220.int.ebiederm.org>
	<87r3w8liw4.fsf@x220.int.ebiederm.org>
	<87iohklfvj.fsf_-_@x220.int.ebiederm.org>
Date: Tue, 09 Dec 2014 14:38:23 -0600
In-Reply-To: <87iohklfvj.fsf_-_@x220.int.ebiederm.org> (Eric W. Biederman's
	message of "Tue, 09 Dec 2014 14:36:48 -0600")
Message-ID: <87d27slfsw.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 2/8] 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 |  5 +++++
 kernel/groups.c                |  4 +++-
 kernel/user_namespace.c        | 14 ++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index e95372654f09..8d493083486a 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -63,6 +63,7 @@ extern const struct seq_operations proc_projid_seq_operations;
 extern ssize_t proc_uid_map_write(struct file *, const char __user *, size_t, loff_t *);
 extern ssize_t proc_gid_map_write(struct file *, const char __user *, size_t, loff_t *);
 extern ssize_t proc_projid_map_write(struct file *, const char __user *, size_t, loff_t *);
+extern bool userns_may_setgroups(const struct user_namespace *ns);
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
@@ -87,6 +88,10 @@ static inline void put_user_ns(struct user_namespace *ns)
 {
 }
 
+static inline bool userns_may_setgroups(const struct user_namespace *ns)
+{
+	return true;
+}
 #endif
 
 #endif /* _LINUX_USER_H */
diff --git a/kernel/groups.c b/kernel/groups.c
index 02d8a251c476..664411f171b5 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 +218,8 @@ bool may_setgroups(void)
 {
 	struct user_namespace *user_ns = current_user_ns();
 
-	return ns_capable(user_ns, CAP_SETGID);
+	return ns_capable(user_ns, CAP_SETGID) &&
+		userns_may_setgroups(user_ns);
 }
 
 /*
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index b99c862a2e3f..27c8dab48c07 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -843,6 +843,20 @@ static bool new_idmap_permitted(const struct file *file,
 	return false;
 }
 
+bool userns_may_setgroups(const struct user_namespace *ns)
+{
+	bool allowed;
+
+	mutex_lock(&id_map_mutex);
+	/* It is not safe to use setgroups until a gid mapping in
+	 * the user namespace has been established.
+	 */
+	allowed = ns->gid_map.nr_extents != 0;
+	mutex_unlock(&id_map_mutex);
+
+	return allowed;
+}
+
 static void *userns_get(struct task_struct *task)
 {
 	struct user_namespace *user_ns;
-- 
1.9.1

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