Code: static void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
	if (atomic_dec_and_test(&ucounts->count)) {
		spin_lock_irqsave(&ucounts_lock, flags);
 		hlist_del_init(&ucounts->node);
		spin_unlock_irqrestore(&ucounts_lock, flags);
 
		kfree(ucounts);
	}
 }

Vulnerability Type: DoS 
CWE ID: CWE-416
Summary: Race condition in kernel/ucount.c in the Linux kernel through 4.10.2 allows local users to cause a denial of service (use-after-free and system crash) or possibly have unspecified other impact via crafted system calls that leverage certain decrement behavior that causes incorrect interaction between put_ucounts and get_ucounts.
Commit Message: ucount: Remove the atomicity from ucount->count

Always increment/decrement ucount->count under the ucounts_lock.  The
increments are there already and moving the decrements there means the
locking logic of the code is simpler.  This simplification in the
locking logic fixes a race between put_ucounts and get_ucounts that
could result in a use-after-free because the count could go zero then
be found by get_ucounts and then be freed by put_ucounts.

A bug presumably this one was found by a combination of syzkaller and
KASAN.  JongWhan Kim reported the syzkaller failure and Dmitry Vyukov
spotted the race in the code.

Cc: stable@vger.kernel.org
Fixes: f6b2db1a3e8d ("userns: Make the count of user namespaces per user")
Reported-by: JongHwan Kim <zzoru007@gmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>