scsi: libiscsi: Initialize iscsi_conn->dd_data only if memory is allocated

In case of an ib_fast_reg_mr allocation failure during iSER setup, the
machine hits a panic because iscsi_conn->dd_data is initialized
unconditionally, even when no memory is allocated (dd_size == 0).  This
leads invalid pointer dereference during connection teardown.

Fix by setting iscsi_conn->dd_data only if memory is actually allocated.

Panic trace:
------------
 iser: iser_create_fastreg_desc: Failed to allocate ib_fast_reg_mr err=-12
 iser: iser_alloc_rx_descriptors: failed allocating rx descriptors / data buffers
 BUG: unable to handle page fault for address: fffffffffffffff8
 RIP: 0010:swake_up_locked.part.5+0xa/0x40
 Call Trace:
  complete+0x31/0x40
  iscsi_iser_conn_stop+0x88/0xb0 [ib_iser]
  iscsi_stop_conn+0x66/0xc0 [scsi_transport_iscsi]
  iscsi_if_stop_conn+0x14a/0x150 [scsi_transport_iscsi]
  iscsi_if_rx+0x1135/0x1834 [scsi_transport_iscsi]
  ? netlink_lookup+0x12f/0x1b0
  ? netlink_deliver_tap+0x2c/0x200
  netlink_unicast+0x1ab/0x280
  netlink_sendmsg+0x257/0x4f0
  ? _copy_from_user+0x29/0x60
  sock_sendmsg+0x5f/0x70

Signed-off-by: Showrya M N <[email protected]>
Signed-off-by: Potnuri Bharat Teja <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Chris Leech <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

Author: Showrya M N

drivers/scsi/libiscsi.c

@@ -3185,7 +3185,8 @@ iscsi_conn_setup(struct iscsi_cls_session *cls_session, int dd_size,
 		return NULL;
 	conn = cls_conn->dd_data;
 
-	conn->dd_data = cls_conn->dd_data + sizeof(*conn);
+	if (dd_size)
+		conn->dd_data = cls_conn->dd_data + sizeof(*conn);
 	conn->session = session;
 	conn->cls_conn = cls_conn;
 	conn->c_stage = ISCSI_CONN_INITIAL_STAGE;