[PATCH] io-wq: Fix UAF when wakeup wqe in hash waitqueue

From: qiang . zhang
Date: Mon May 24 2021 - 03:33:33 EST

From: Zqiang <qiang.zhang@xxxxxxxxxxxxx>

The syzbot report a UAF when iou-wrk accessing wqe of the hash
waitqueue. in the case of sharing a hash waitqueue between two
io-wq, when one of the io-wq is destroyed, all iou-wrk in this
io-wq are awakened, all wqe belonging to this io-wq are removed
from hash waitqueue, after that, all iou-wrk belonging to this
io-wq begin running, suppose following scenarios, wqe[0] and wqe[1]
belong to this io-wq, and these work has same hash value.

iou-wrk0(wqe[0]) iou-wrk1(wqe[1])

while test_bit IO_WQ_BIT_EXIT while test_bit IO_WQ_BIT_EXIT
schedule_timeout (sleep be break by wakeup io_get_next_work
and the IO_WQ_BIT_EXIT be set) set_bit hash

test_bit IO_WQ_BIT_EXIT (return true)
wqe->work_list (is not empty)
test_and_set_bit hash (is true) (hash!=-1U&&!next_hashed) true
(there is no work other than hash work)
io_wait_on_hash clear_bit hash
spin_lock wq_has_sleeper (is false)
list_empty(&wqe->wait.entry) (is true)
__add_wait_queue (hash->wait is empty,not wakeup
and IO_WQ_BIT_EXIT has been set,
........ the wqe->work_list is empty exit
(there is no work other than hash work while loop)
io_get_next_work will return NULL)
return NULL (the wqe->work_list is empty
the io_worker_handle_work is not
io_worker_exit io_worker_exit

In the above scenario, wqe may be mistakenly removing
opportunities from the queue, this leads to when the wqe is
released, it still in hash waitqueue. when a iou-wrk belonging
to another io-wq access hash waitqueue will trigger UAF,
To avoid this phenomenon, after all iou-wrk thread belonging to the
io-wq exit, remove wqe from the hash waiqueue, at this time,
there will be no operation to queue the wqe.

Reported-by: syzbot+6cb11ade52aa17095297@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
fs/io-wq.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 5361a9b4b47b..911a1274aabd 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -1003,13 +1003,16 @@ static void io_wq_exit_workers(struct io_wq *wq)
struct io_wqe *wqe = wq->wqes[node];

io_wq_for_each_worker(wqe, io_wq_worker_wake, NULL);
- spin_lock_irq(&wq->hash->wait.lock);
- list_del_init(&wq->wqes[node]->wait.entry);
- spin_unlock_irq(&wq->hash->wait.lock);
+ for_each_node(node) {
+ spin_lock_irq(&wq->hash->wait.lock);
+ list_del_init(&wq->wqes[node]->wait.entry);
+ spin_unlock_irq(&wq->hash->wait.lock);
+ }
wq->task = NULL;