Merge branch 'for-v5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

6209049e · Linus Torvalds <[email protected]> · 2021-08-05 12:00:00 -07:00

Pull ucounts fix from Eric Biederman:
 "Fix a subtle locking versus reference counting bug in the ucount
  changes, found by syzbot"

* 'for-v5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  ucounts: Fix race condition between alloc_ucounts and put_ucounts

Commit: 6209049ecfc1894453d1fc850e60c58d4eccaf2a
Parents: 3c3e9027 · 345daff2

Merge commit. Changes below are compared with the first parent: 3c3e9027.

Changes

1 files changed · +7 −3

File	Status	Add	Del	Content
kernel/ucount.c	modified	+7	−3	View file

kernel/ucount.c

View file

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 87799e2..77be3bb 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -160,6 +160,7 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
 	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
 	struct ucounts *ucounts, *new;
+	long overflow;
 
 	spin_lock_irq(&ucounts_lock);
 	ucounts = find_ucounts(ns, uid, hashent);
@@ -184,8 +185,12 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 			return new;
 		}
 	}
+	overflow = atomic_add_negative(1, &ucounts->count);
 	spin_unlock_irq(&ucounts_lock);
-	ucounts = get_ucounts(ucounts);
+	if (overflow) {
+		put_ucounts(ucounts);
+		return NULL;
+	}
 	return ucounts;
 }
 
@@ -193,8 +198,7 @@ void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
-	if (atomic_dec_and_test(&ucounts->count)) {
-		spin_lock_irqsave(&ucounts_lock, flags);
+	if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
 		hlist_del_init(&ucounts->node);
 		spin_unlock_irqrestore(&ucounts_lock, flags);
 		kfree(ucounts);