scsi: libiscsi: fix possible NULL pointer dereference in case of TMF

In iscsi_check_tmf_restrictions() task->hdr is dereferenced to print the
opcode, it is possible that task->hdr is NULL.

There are two cases based on opcode argument:

1. ISCSI_OP_SCSI_CMD - In this case alloc_pdu() is called
after iscsi_check_tmf_restrictions()

iscsi_prep_scsi_cmd_pdu() -> iscsi_check_tmf_restrictions() -> alloc_pdu().

Transport drivers allocate memory for iSCSI hdr in alloc_pdu() and assign
it to task->hdr. In case of TMF task->hdr will be NULL resulting in NULL
pointer dereference.

2. ISCSI_OP_SCSI_DATA_OUT - In this case transport driver can free the
memory for iSCSI hdr after transmitting the pdu so task->hdr can be NULL or
invalid.

This patch fixes this issue by removing task->hdr->opcode from the printk
statement.
Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libiscsi.c

@@ -284,11 +284,11 @@ static int iscsi_check_tmf_restrictions(struct iscsi_task *task, int opcode)
 		 */
 		if (opcode != ISCSI_OP_SCSI_DATA_OUT) {
 			iscsi_conn_printk(KERN_INFO, conn,
-					  "task [op %x/%x itt "
+					  "task [op %x itt "
 					  "0x%x/0x%x] "
 					  "rejected.\n",
-					  task->hdr->opcode, opcode,
-					  task->itt, task->hdr_itt);
+					  opcode, task->itt,
+					  task->hdr_itt);
 			return -EACCES;
 		}
 		/*
@@ -297,10 +297,10 @@ static int iscsi_check_tmf_restrictions(struct iscsi_task *task, int opcode)
 		 */
 		if (conn->session->fast_abort) {
 			iscsi_conn_printk(KERN_INFO, conn,
-					  "task [op %x/%x itt "
+					  "task [op %x itt "
 					  "0x%x/0x%x] fast abort.\n",
-					  task->hdr->opcode, opcode,
-					  task->itt, task->hdr_itt);
+					  opcode, task->itt,
+					  task->hdr_itt);
 			return -EACCES;
 		}
 		break;