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@xxxxxxxxxxxxxxx
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
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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