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 --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index be765234c0a2b999f0f2bfcdb9f36b2cc16fa0c6..32354b4b4b2ba5ae72034d00c3b1d43fa8c2a15c 100644 (file)
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -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];
 };
 

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 62630a40ab3a4225291c7804ff67917aa997646c..b4eeee03934fe8f083b70e9907be0721759bc3be 100644 (file)
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -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;
 }
@@ -165,13 +167,15 @@ 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)