[PATCH review 10/18] userns: Don't allow setgroups until a gid mapping has been setablished

Eric W. Biederman
2014-12-12T22:48:21Z

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;
--
1.9.1