CVE-2014-8989 userns can bypass group restrictions (rhbz 1170684 1170688)

1 files changed, 98 insertions, 0 deletions

diff --git a/userns-Don-t-allow-setgroups-until-a-gid-mapping-has.patch b/userns-Don-t-allow-setgroups-until-a-gid-mapping-has.patch
new file mode 100644
index 000000000..81217d2a5
--- /dev/null
+++ b/userns-Don-t-allow-setgroups-until-a-gid-mapping-has.patch
@@ -0,0 +1,98 @@
+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
+