io_uring/zcrx: use READ_ONCE with user shared RQEs

Refill queue entries are shared with the user space, use READ_ONCE when
reading them.

Fixes: 34a3e60 ("io_uring/zcrx: implement zerocopy receive pp memory provider");
Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -927,11 +927,12 @@ static inline bool io_parse_rqe(struct io_uring_zcrx_rqe *rqe,
 				struct io_zcrx_ifq *ifq,
 				struct net_iov **ret_niov)
 {
+	__u64 off = READ_ONCE(rqe->off);
 	unsigned niov_idx, area_idx;
 	struct io_zcrx_area *area;
 
-	area_idx = rqe->off >> IORING_ZCRX_AREA_SHIFT;
-	niov_idx = (rqe->off & ~IORING_ZCRX_AREA_MASK) >> ifq->niov_shift;
+	area_idx = off >> IORING_ZCRX_AREA_SHIFT;
+	niov_idx = (off & ~IORING_ZCRX_AREA_MASK) >> ifq->niov_shift;
 
 	if (unlikely(rqe->__pad || area_idx))
 		return false;