scsi: target: Fix NULL dereference on XCOPY completion

CPU affinity control added with commit 39ae3ed ("scsi: target: core:
Make completion affinity configurable") makes target_complete_cmd() queue
work on a CPU based on se_tpg->se_tpg_wwn->cmd_compl_affinity state.

LIO's EXTENDED COPY worker is a special case in that read/write cmds are
dispatched using the global xcopy_pt_tpg, which carries a NULL se_tpg_wwn
pointer following initialization in target_xcopy_setup_pt().

The NULL xcopy_pt_tpg->se_tpg_wwn pointer is dereferenced on completion of
any EXTENDED COPY initiated read/write cmds. E.g using the libiscsi
SCSI.ExtendedCopy.Simple test:

  BUG: kernel NULL pointer dereference, address: 00000000000001a8
  RIP: 0010:target_complete_cmd+0x9d/0x130 [target_core_mod]
  Call Trace:
   fd_execute_rw+0x148/0x42a [target_core_file]
   ? __dynamic_pr_debug+0xa7/0xe0
   ? target_check_reservation+0x5b/0x940 [target_core_mod]
   __target_execute_cmd+0x1e/0x90 [target_core_mod]
   transport_generic_new_cmd+0x17c/0x330 [target_core_mod]
   target_xcopy_issue_pt_cmd+0x9/0x60 [target_core_mod]
   target_xcopy_read_source.isra.7+0x10b/0x1b0 [target_core_mod]
   ? target_check_fua+0x40/0x40 [target_core_mod]
   ? transport_complete_task_attr+0x130/0x130 [target_core_mod]
   target_xcopy_do_work+0x61f/0xc00 [target_core_mod]

This fix makes target_complete_cmd() queue work on se_cmd->cpuid if
se_tpg_wwn is NULL.

Link: https://lore.kernel.org/r/[email protected]
Fixes: 39ae3ed ("scsi: target: core: Make completion affinity configurable")
Cc: Lee Duncan <[email protected]>
Cc: Mike Christie <[email protected]>
Reviewed-by: Mike Christie <[email protected]>
Signed-off-by: David Disseldorp <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

Author: ddiss

drivers/target/target_core_transport.c

@@ -886,7 +886,7 @@ void target_complete_cmd(struct se_cmd *cmd, u8 scsi_status)
 	INIT_WORK(&cmd->work, success ? target_complete_ok_work :
 		  target_complete_failure_work);
 
-	if (wwn->cmd_compl_affinity == SE_COMPL_AFFINITY_CPUID)
+	if (!wwn || wwn->cmd_compl_affinity == SE_COMPL_AFFINITY_CPUID)
 		cpu = cmd->cpuid;
 	else
 		cpu = wwn->cmd_compl_affinity;