[PATCH] io_uring/net: fix iter retargeting for selected buf

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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@xxxxxxxxxxxxxxx
Reported-by: syzbot+a4c6e5ef999b68b26ed1@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 0455d4ccec548 ("io_uring: add POLL_FIRST support for send/sendmsg and recv/recvmsg")
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>

Can be cleaned up if we rely on UBUF, but it's easier to backport
this way

 io_uring/net.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/io_uring/net.c b/io_uring/net.c
index 3d07bf79c1e0..7a8e298af81b 100644
--- a/io_uring/net.c
+++ b/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;
 	if (req->ctx->compat)

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