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io_uring: fix UAF due to missing POLLFREE handling

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Fixes a problem described in 50252e4b5e
("aio: fix use-after-free due to missing POLLFREE handling")
and copies the approach used there.

In short, we have to forcibly eject a poll entry when we meet POLLFREE.
We can't rely on io_poll_get_ownership() as can't wait for potentially
running tw handlers, so we use the fact that wqs are RCU freed. See
Eric's patch and comments for more details.

Reported-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209010455.42744-6-ebiggers@kernel.org
Reported-and-tested-by: syzbot+5426c7ed6868c705ca14@syzkaller.appspotmail.com
Fixes: 221c5eb233 ("io_uring: add support for IORING_OP_POLL")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4ed56b6f548f7ea337603a82315750449412748a.1642161259.git.asml.silence@gmail.com
[axboe: drop non-functional change from patch]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2022-01-14 11:59:10 +00:00 committed by Jens Axboe
parent c84b8a3fef
commit 791f3465c4
1 changed files with 50 additions and 8 deletions
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58
fs/io_uring.c
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@ -5462,12 +5462,14 @@ static void io_init_poll_iocb(struct io_poll_iocb *poll, __poll_t events,
static inline void io_poll_remove_entry(struct io_poll_iocb *poll)
{
struct wait_queue_head *head = poll->head;
struct wait_queue_head *head = smp_load_acquire(&poll->head);
spin_lock_irq(&head->lock);
list_del_init(&poll->wait.entry);
poll->head = NULL;
spin_unlock_irq(&head->lock);
if (head) {
spin_lock_irq(&head->lock);
list_del_init(&poll->wait.entry);
poll->head = NULL;
spin_unlock_irq(&head->lock);
}
}
static void io_poll_remove_entries(struct io_kiocb *req)
@ -5475,10 +5477,26 @@ static void io_poll_remove_entries(struct io_kiocb *req)
struct io_poll_iocb *poll = io_poll_get_single(req);
struct io_poll_iocb *poll_double = io_poll_get_double(req);
if (poll->head)
io_poll_remove_entry(poll);
if (poll_double && poll_double->head)
/*
* While we hold the waitqueue lock and the waitqueue is nonempty,
* wake_up_pollfree() will wait for us. However, taking the waitqueue
* lock in the first place can race with the waitqueue being freed.
*
* We solve this as eventpoll does: by taking advantage of the fact that
* all users of wake_up_pollfree() will RCU-delay the actual free. If
* we enter rcu_read_lock() and see that the pointer to the queue is
* non-NULL, we can then lock it without the memory being freed out from
* under us.
*
* Keep holding rcu_read_lock() as long as we hold the queue lock, in
* case the caller deletes the entry from the queue, leaving it empty.
* In that case, only RCU prevents the queue memory from being freed.
*/
rcu_read_lock();
io_poll_remove_entry(poll);
if (poll_double)
io_poll_remove_entry(poll_double);
rcu_read_unlock();
}
/*
@ -5618,6 +5636,30 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
wait);
__poll_t mask = key_to_poll(key);
if (unlikely(mask & POLLFREE)) {
io_poll_mark_cancelled(req);
/* we have to kick tw in case it's not already */
io_poll_execute(req, 0);
/*
* If the waitqueue is being freed early but someone is already
* holds ownership over it, we have to tear down the request as
* best we can. That means immediately removing the request from
* its waitqueue and preventing all further accesses to the
* waitqueue via the request.
*/
list_del_init(&poll->wait.entry);
/*
* Careful: this *must* be the last step, since as soon
* as req->head is NULL'ed out, the request can be
* completed and freed, since aio_poll_complete_work()
* will no longer need to take the waitqueue lock.
*/
smp_store_release(&poll->head, NULL);
return 1;
}
/* for instances that support it check for an event match first */
if (mask && !(mask & poll->events))
return 0;