From: "Eric W. Biederman" <ebiederm@xmission.com>

Date: Fri, 5 Dec 2014 18:01:11 -0600

Subject: [PATCH] userns: Don't allow setgroups until a gid mapping has been

 setablished

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

Reviewed-by: Andy Lutomirski <luto@amacapital.net>

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;

-- 

2.1.0