io_uring/net: fix iter retargeting for selected buf

When using selected buffer feature, io_uring delays data iter setup
until later. If io_setup_async_msg() is called before that it might see
not correctly setup iterator. Pre-init nr_segs and judge from its state
whether we repointing.

Cc: stable@vger.kernel.org
Reported-by: syzbot+a4c6e5ef999b68b26ed1@syzkaller.appspotmail.com
Fixes: 0455d4ccec ("io_uring: add POLL_FIRST support for send/sendmsg and recv/recvmsg")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0000000000002770be06053c7757@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/net.c

@@ -183,6 +183,10 @@ static int io_setup_async_msg(struct io_kiocb *req,
 	memcpy(async_msg, kmsg, sizeof(*kmsg));
 	if (async_msg->msg.msg_name)
 		async_msg->msg.msg_name = &async_msg->addr;
+
+	if ((req->flags & REQ_F_BUFFER_SELECT) && !async_msg->msg.msg_iter.nr_segs)
+		return -EAGAIN;
+
 	/* if were using fast_iov, set it to the new one */
 	if (iter_is_iovec(&kmsg->msg.msg_iter) && !kmsg->free_iov) {
 		size_t fast_idx = iter_iov(&kmsg->msg.msg_iter) - kmsg->fast_iov;
@@ -542,6 +546,7 @@ static int io_recvmsg_copy_hdr(struct io_kiocb *req,
 			       struct io_async_msghdr *iomsg)
 {
 	iomsg->msg.msg_name = &iomsg->addr;
+	iomsg->msg.msg_iter.nr_segs = 0;
 
 #ifdef CONFIG_COMPAT
 	if (req->ctx->compat)