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Commit 040757f7 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

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: f6b2db1a ("userns: Make the count of user namespaces per user")
Reported-by: default avatarJongHwan Kim <zzoru007@gmail.com>
Reported-by: default avatarDmitry Vyukov <dvyukov@google.com>
Reviewed-by: default avatarAndrei Vagin <avagin@gmail.com>
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent c1ae3cfa

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)

{