scsi: iscsi: target: Don't use stack buffer for scatterlist

Fedora got a bug report of a crash with iSCSI:

kernel BUG at include/linux/scatterlist.h:143!
...
RIP: 0010:iscsit_do_crypto_hash_buf+0x154/0x180 [iscsi_target_mod]
...
 Call Trace:
  ? iscsi_target_tx_thread+0x200/0x200 [iscsi_target_mod]
  iscsit_get_rx_pdu+0x4cd/0xa90 [iscsi_target_mod]
  ? native_sched_clock+0x3e/0xa0
  ? iscsi_target_tx_thread+0x200/0x200 [iscsi_target_mod]
  iscsi_target_rx_thread+0x81/0xf0 [iscsi_target_mod]
  kthread+0x120/0x140
  ? kthread_create_worker_on_cpu+0x70/0x70
  ret_from_fork+0x3a/0x50

This is a BUG_ON for using a stack buffer with a scatterlist.  There
are two cases that trigger this bug. Switch to using a dynamically
allocated buffer for one case and do not assign a NULL buffer in
another case.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/iscsi/iscsi_target.c

@@ -1416,7 +1416,8 @@ static void iscsit_do_crypto_hash_buf(struct ahash_request *hash,
 
 	sg_init_table(sg, ARRAY_SIZE(sg));
 	sg_set_buf(sg, buf, payload_length);
-	sg_set_buf(sg + 1, pad_bytes, padding);
+	if (padding)
+		sg_set_buf(sg + 1, pad_bytes, padding);
 
 	ahash_request_set_crypt(hash, sg, data_crc, payload_length + padding);
 
@@ -3910,18 +3911,21 @@ static bool iscsi_target_check_conn_state(struct iscsi_conn *conn)
 static void iscsit_get_rx_pdu(struct iscsi_conn *conn)
 {
 	int ret;
-	u8 buffer[ISCSI_HDR_LEN], opcode;
+	u8 *buffer, opcode;
 	u32 checksum = 0, digest = 0;
 	struct kvec iov;
 
+	buffer = kcalloc(ISCSI_HDR_LEN, sizeof(*buffer), GFP_KERNEL);
+	if (!buffer)
+		return;
+
 	while (!kthread_should_stop()) {
 		/*
 		 * Ensure that both TX and RX per connection kthreads
 		 * are scheduled to run on the same CPU.
 		 */
 		iscsit_thread_check_cpumask(conn, current, 0);
 
-		memset(buffer, 0, ISCSI_HDR_LEN);
 		memset(&iov, 0, sizeof(struct kvec));
 
 		iov.iov_base	= buffer;
@@ -3930,7 +3934,7 @@ static void iscsit_get_rx_pdu(struct iscsi_conn *conn)
 		ret = rx_data(conn, &iov, 1, ISCSI_HDR_LEN);
 		if (ret != ISCSI_HDR_LEN) {
 			iscsit_rx_thread_wait_for_tcp(conn);
-			return;
+			break;
 		}
 
 		if (conn->conn_ops->HeaderDigest) {
@@ -3940,7 +3944,7 @@ static void iscsit_get_rx_pdu(struct iscsi_conn *conn)
 			ret = rx_data(conn, &iov, 1, ISCSI_CRC_LEN);
 			if (ret != ISCSI_CRC_LEN) {
 				iscsit_rx_thread_wait_for_tcp(conn);
-				return;
+				break;
 			}
 
 			iscsit_do_crypto_hash_buf(conn->conn_rx_hash, buffer,
@@ -3964,7 +3968,7 @@ static void iscsit_get_rx_pdu(struct iscsi_conn *conn)
 		}
 
 		if (conn->conn_state == TARG_CONN_STATE_IN_LOGOUT)
-			return;
+			break;
 
 		opcode = buffer[0] & ISCSI_OPCODE_MASK;
 
@@ -3975,13 +3979,15 @@ static void iscsit_get_rx_pdu(struct iscsi_conn *conn)
 			" while in Discovery Session, rejecting.\n", opcode);
 			iscsit_add_reject(conn, ISCSI_REASON_PROTOCOL_ERROR,
 					  buffer);
-			return;
+			break;
 		}
 
 		ret = iscsi_target_rx_opcode(conn, buffer);
 		if (ret < 0)
-			return;
+			break;
 	}
+
+	kfree(buffer);
 }
 
 int iscsi_target_rx_thread(void *arg)