io_uring/net: fix sr->len for IORING_OP_RECV with MSG_WAITALL and buffers

If we use IORING_OP_RECV with provided buffers and pass in '0' as the
length of the request, the length is retrieved from the selected buffer.
If MSG_WAITALL is also set and we get a short receive, then we may hit
the retry path which decrements sr->len and increments the buffer for
a retry. However, the length is still zero at this point, which means
that sr->len now becomes huge and import_ubuf() will cap it to
MAX_RW_COUNT and subsequently return -EFAULT for the range as a whole.

Fix this by always assigning sr->len once the buffer has been selected.

Cc: stable@vger.kernel.org
Fixes: 7ba89d2af1 ("io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/net.c

@@ -923,6 +923,7 @@ retry_multishot:
 		if (!buf)
 			return -ENOBUFS;
 		sr->buf = buf;
+		sr->len = len;
 	}
 
 	ret = import_ubuf(ITER_DEST, sr->buf, len, &msg.msg_iter);