[PATCH 4.10 18/48] ucount: Remove the atomicity from ucount->count

Greg Kroah-Hartman
2017-03-16T14:36:22Z

From: Eric W. Biederman <ebiederm@xmission.com>

commit 040757f738e13caaa9c5078bca79aa97e11dde88 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: 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/user_namespace.h | 2 +-
kernel/ucount.c | 18 +++++++++++-------
2 files changed, 12 insertions(+), 8 deletions(-)

--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -65,7 +65,7 @@ struct ucounts {
struct hlist_node node;
struct user_namespace *ns;
kuid_t uid;
- atomic_t count;
+ int count;
atomic_t ucount[UCOUNT_COUNTS];
};

--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struc

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);
@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struc
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;
}
@@ -160,13 +162,15 @@ static void put_ucounts(struct 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)