io_uring: fix use-after-free of shadow_req

There is a potential dangling pointer problem. we never clean
shadow_req, if there are multiple link lists in this series of
sqes, then the shadow_req will not reallocate, and continue to
use the last one. but in the previous, his memory has been
released, thus forming a dangling pointer. let's clean up him
and make sure that every new link list can reapply for a new
shadow_req.

Fixes: 4fe2c963 ("io_uring: add support for link with drain")
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2357,6 +2357,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, struct sqe_submit *sqes,
 			io_queue_link_head(ctx, link, &link->submit, shadow_req,
 						true);
 			link = NULL;
+			shadow_req = NULL;
 		}
 		prev_was_link = (sqes[i].sqe->flags & IOSQE_IO_LINK) != 0;
 
@@ -2543,6 +2544,7 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit,
 			io_queue_link_head(ctx, link, &link->submit, shadow_req,
 						force_nonblock);
 			link = NULL;
+			shadow_req = NULL;
 		}
 		prev_was_link = (s.sqe->flags & IOSQE_IO_LINK) != 0;