CVE-2017-6874: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') (CWE-362)
value: 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.
CWE description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
CVSS v3 base score: 7
Published: 2017-03-14T09:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: include/linux/user_namespace.h, kernel/ucount.c
Commit date: 2017-03-06 15:26:37 -0600
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>

Diff: diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index be765234c0a2..32354b4b4b2b 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -70,11 +70,11 @@ struct user_namespace {
 
 struct ucounts {
 	struct hlist_node node;
 	struct user_namespace *ns;
 	kuid_t uid;
-	atomic_t count;
+	int count;
 	atomic_t ucount[UCOUNT_COUNTS];
 };
 
 extern struct user_namespace init_user_ns;
 
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 62630a40ab3a..b4eeee03934f 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -142,38 +142,42 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
 		if (!new)
 			return NULL;
 
 		new->ns = ns;
 		new->uid = uid;
-		atomic_set(&new->count, 0);
+		new->count = 0;
 
 		spin_lock_irq(&ucounts_lock);
 		ucounts = find_ucounts(ns, uid, hashent);
 		if (ucounts) {
 			kfree(new);
 		} else {
 			hlist_add_head(&new->node, hashent);
 			ucounts = new;
 		}
 	}
-	if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
+	if (ucounts->count == INT_MAX)
 		ucounts = NULL;
+	else
+		ucounts->count += 1;
 	spin_unlock_irq(&ucounts_lock);
 	return ucounts;
 }
 
 static void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
-	if (atomic_dec_and_test(&ucounts->count)) {
-		spin_lock_irqsave(&ucounts_lock, flags);
+	spin_lock_irqsave(&ucounts_lock, flags);
+	ucounts->count -= 1;
+	if (!ucounts->count)
 		hlist_del_init(&ucounts->node);
-		spin_unlock_irqrestore(&ucounts_lock, flags);
+	else
+		ucounts = NULL;
+	spin_unlock_irqrestore(&ucounts_lock, flags);
 
-		kfree(ucounts);
-	}
+	kfree(ucounts);
 }
 
 static inline bool atomic_inc_below(atomic_t *v, int u)
 {
 	int c, old;

Vulnerable code: 	atomic_t count;
		atomic_set(&new->count, 0);
	if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
	if (atomic_dec_and_test(&ucounts->count)) {
		spin_lock_irqsave(&ucounts_lock, flags);
		spin_unlock_irqrestore(&ucounts_lock, flags);
		kfree(ucounts);
	}
Fixed code: 	int count;
		new->count = 0;
	if (ucounts->count == INT_MAX)
	else
		ucounts->count += 1;
	spin_lock_irqsave(&ucounts_lock, flags);
	ucounts->count -= 1;
	if (!ucounts->count)
	else
		ucounts = NULL;
	spin_unlock_irqrestore(&ucounts_lock, flags);
	kfree(ucounts);