Commit ee6f7ee1 authored by Eric W. Biederman's avatar Eric W. Biederman Committed by Greg Kroah-Hartman

ucount: Remove the atomicity from ucount->count

commit 040757f7 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.

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>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 8bb208d0
...@@ -65,7 +65,7 @@ struct ucounts { ...@@ -65,7 +65,7 @@ struct ucounts {
struct hlist_node node; struct hlist_node node;
struct user_namespace *ns; struct user_namespace *ns;
kuid_t uid; kuid_t uid;
atomic_t count; int count;
atomic_t ucount[UCOUNT_COUNTS]; atomic_t ucount[UCOUNT_COUNTS];
}; };
......
...@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid) ...@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
new->ns = ns; new->ns = ns;
new->uid = uid; new->uid = uid;
atomic_set(&new->count, 0); new->count = 0;
spin_lock_irq(&ucounts_lock); spin_lock_irq(&ucounts_lock);
ucounts = find_ucounts(ns, uid, hashent); ucounts = find_ucounts(ns, uid, hashent);
...@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid) ...@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
ucounts = new; ucounts = new;
} }
} }
if (!atomic_add_unless(&ucounts->count, 1, INT_MAX)) if (ucounts->count == INT_MAX)
ucounts = NULL; ucounts = NULL;
else
ucounts->count += 1;
spin_unlock_irq(&ucounts_lock); spin_unlock_irq(&ucounts_lock);
return ucounts; return ucounts;
} }
...@@ -160,13 +162,15 @@ static void put_ucounts(struct ucounts *ucounts) ...@@ -160,13 +162,15 @@ static void put_ucounts(struct ucounts *ucounts)
{ {
unsigned long flags; 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); 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) static inline bool atomic_inc_below(atomic_t *v, int u)
......
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