nvmet-tcp: fix out-of-bounds write when receiving an over-long PDU

nvmet_tcp_try_recv_pdu() reads a PDU header into the fixed 128-byte
queue->pdu union, then computes the remaining payload length as

	queue->left = hdr->hlen - queue->offset + hdgst;

and reads that many more bytes into &queue->pdu + queue->offset, without
ever bounding the result against sizeof(queue->pdu).

A struct nvme_tcp_icreq_pdu is itself 128 bytes, exactly the size of the
union. Once a header digest has been negotiated (hdgst = 4), a second
ICReq passes the hlen == nvmet_tcp_pdu_size() check but yields
queue->left = 128 - 8 + 4 = 124, so bytes 8..132 are written into the
128-byte buffer -- 4 bytes past its end, over queue->hdr_digest and
queue->data_digest. Those bytes are attacker-controlled (an ICReq
carries no digest), and the duplicate ICReq is only rejected later,
after the overflow. A remote unauthenticated host can thus corrupt
kernel memory adjacent to the receive buffer.

Reject any PDU whose declared length would read past the end of
queue->pdu before the second recv.

Fixes: 872d26a391 ("nvmet-tcp: add NVMe over TCP target driver")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Shivam Kumar <kumar.shivam43666@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/target/tcp.c

@@ -1244,6 +1244,8 @@ static int nvmet_tcp_try_recv_pdu(struct nvmet_tcp_queue *queue)
 		}
 
 		queue->left = hdr->hlen - queue->offset + hdgst;
+		if (queue->left > sizeof(queue->pdu) - queue->offset)
+			return -EPROTO;
 		goto recv;
 	}