io_uring: fix ltout double free on completion race

Always remove linked timeout on io_link_timeout_fn() from the master
request link list, otherwise we may get use-after-free when first
io_link_timeout_fn() puts linked timeout in the fail path, and then
will be found and put on master's free.

Cc: stable@vger.kernel.org # 5.10+
Fixes: 90cd7e424969d ("io_uring: track link timeout's master explicitly")
Reported-and-tested-by: syzbot+5a864149dd970b546223@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/69c46bf6ce37fec4fdcd98f0882e18eb07ce693a.1620990121.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9ac5e27..599102c 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6354,10 +6354,11 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
         * We don't expect the list to be empty, that will only happen if we
         * race with the completion of the linked work.
         */
-       if (prev && req_ref_inc_not_zero(prev))
+       if (prev) {
                io_remove_next_linked(prev);
-       else
-               prev = NULL;
+               if (!req_ref_inc_not_zero(prev))
+                       prev = NULL;
+       }
        spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
        if (prev) {