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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>
This commit is contained in:
Eric W. Biederman 2014-12-05 18:01:11 -06:00
parent 0542f17bf2
commit 273d2c67c3
3 changed files with 22 additions and 1 deletions
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5
include/linux/user_namespace.h
View File

@ -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 */

4
kernel/groups.c
View File

@ -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);
}
/*

14
kernel/user_namespace.c
View File

@ -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;