ucount: Remove the atomicity from ucount->count (040757f7) · Commits · 戴 / test

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: ("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>

include/linux/user_namespace.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -72,7 +72,7 @@ struct ucounts {
		struct hlist_node node;
		struct user_namespace *ns;
		kuid_t uid;
		atomic_t count;
		int count;
		atomic_t ucount[UCOUNT_COUNTS];
		};

kernel/ucount.c

+11 −7

Original line number	Diff line number	Diff line
		@@ -144,7 +144,7 @@ static struct ucounts get_ucounts(struct user_namespace ns, kuid_t uid)

		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);
		@@ -155,8 +155,10 @@ static struct ucounts get_ucounts(struct user_namespace ns, kuid_t uid)
		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;
		}
		@@ -165,14 +167,16 @@ static void put_ucounts(struct ucounts *ucounts)
		{
		unsigned long flags;

		if (atomic_dec_and_test(&ucounts->count)) {
		spin_lock_irqsave(&ucounts_lock, flags);
		ucounts->count -= 1;
		if (!ucounts->count)
		hlist_del_init(&ucounts->node);
		else
		ucounts = NULL;
		spin_unlock_irqrestore(&ucounts_lock, flags);

		kfree(ucounts);
		}
		}

		static inline bool atomic_inc_below(atomic_t *v, int u)
		{

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