The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 791f3465c4afde02d7f16cf7424ca87070b69396 Mon Sep 17 00:00:00 2001 From: Pavel Begunkov asml.silence@gmail.com Date: Fri, 14 Jan 2022 11:59:10 +0000 Subject: [PATCH] io_uring: fix UAF due to missing POLLFREE handling

Fixes a problem described in 50252e4b5e989 ("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: 221c5eb233823 ("io_uring: add support for IORING_OP_POLL") Signed-off-by: Pavel Begunkov asml.silence@gmail.com Link: https://lore.kernel.org/r/4ed56b6f548f7ea337603a82315750449412748a.164216125... [axboe: drop non-functional change from patch] Signed-off-by: Jens Axboe axboe@kernel.dk

diff --git a/fs/io_uring.c b/fs/io_uring.c index fa3277844d2e..422d6de48688 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -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;