Patch "ucount: Remove the atomicity from ucount->count" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

    ucount: Remove the atomicity from ucount->count

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ucount-remove-the-atomicity-from-ucount-count.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Sun, 5 Mar 2017 15:03:22 -0600
Subject: ucount: Remove the atomicity from ucount->count

From: Eric W. Biederman <[email protected]>

commit 040757f738e13caaa9c5078bca79aa97e11dde88 upstream.

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.

---
 include/linux/user_namespace.h |    2 +-
 kernel/ucount.c                |   18 +++++++++++-------
 2 files changed, 12 insertions(+), 8 deletions(-)

@@ -65,7 +65,7 @@ struct ucounts {
 	struct hlist_node node;
 	struct user_namespace *ns;
 	kuid_t uid;
-	atomic_t count;
+	int count;
 	atomic_t ucount[UCOUNT_COUNTS];
 };
 

@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struc
 
 		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);

@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struc
 			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;
 }

@@ -160,13 +162,15 @@ static void put_ucounts(struct 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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ucount-remove-the-atomicity-from-ucount-count.patch