nvme-tcp: fix NULL pointer dereferences in nvmet_tcp_build_pdu_iovec

Commit efa5630 ("nvmet-tcp: Fix a kernel panic when host sends an invalid H2C PDU length")
added ttag bounds checking and data_offset
validation in nvmet_tcp_handle_h2c_data_pdu(), but it did not validate
whether the command's data structures (cmd->req.sg and cmd->iov) have
been properly initialized before processing H2C_DATA PDUs.

The nvmet_tcp_build_pdu_iovec() function dereferences these pointers
without NULL checks. This can be triggered by sending H2C_DATA PDU
immediately after the ICREQ/ICRESP handshake, before
sending a CONNECT command or NVMe write command.

Attack vectors that trigger NULL pointer dereferences:
1. H2C_DATA PDU sent before CONNECT → both pointers NULL
2. H2C_DATA PDU for READ command → cmd->req.sg allocated, cmd->iov NULL
3. H2C_DATA PDU for uninitialized command slot → both pointers NULL

The fix validates both cmd->req.sg and cmd->iov before calling
nvmet_tcp_build_pdu_iovec(). Both checks are required because:
- Uninitialized commands: both NULL
- READ commands: cmd->req.sg allocated, cmd->iov NULL
- WRITE commands: both allocated

Fixes: efa5630 ("nvmet-tcp: Fix a kernel panic when host sends an invalid H2C PDU length")
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Shivam Kumar <kumar.shivam43666@gmail.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

Author: shivam-2315

drivers/nvme/target/tcp.c

@@ -982,6 +982,18 @@ static int nvmet_tcp_handle_h2c_data_pdu(struct nvmet_tcp_queue *queue)
 		pr_err("H2CData PDU len %u is invalid\n", cmd->pdu_len);
 		goto err_proto;
 	}
+       /*
+	* Ensure command data structures are initialized. We must check both
+	* cmd->req.sg and cmd->iov because they can have different NULL states:
+	* - Uninitialized commands: both NULL
+	* - READ commands: cmd->req.sg allocated, cmd->iov NULL
+	* - WRITE commands: both allocated
+	*/
+	if (unlikely(!cmd->req.sg || !cmd->iov)) {
+		pr_err("queue %d: H2CData PDU received for invalid command state (ttag %u)\n",
+			queue->idx, data->ttag);
+		goto err_proto;
+	}
 	cmd->pdu_recv = 0;
 	nvmet_tcp_build_pdu_iovec(cmd);
 	queue->cmd = cmd;