scsi: zfcp: Fix use-after-free in request timeout handlers

Before v4.15 commit 75492a51 ("s390/scsi: Convert timers to use
timer_setup()"), we intentionally only passed zfcp_adapter as context
argument to zfcp_fsf_request_timeout_handler(). Since we only trigger
adapter recovery, it was unnecessary to sync against races between timeout
and (late) completion.  Likewise, we only passed zfcp_erp_action as context
argument to zfcp_erp_timeout_handler(). Since we only wakeup an ERP action,
it was unnecessary to sync against races between timeout and (late)
completion.

Meanwhile the timeout handlers get timer_list as context argument and do a
timer-specific container-of to zfcp_fsf_req which can have been freed.

Fix it by making sure that any request timeout handlers, that might just
have started before del_timer(), are completed by using del_timer_sync()
instead. This ensures the request free happens afterwards.

Space time diagram of potential use-after-free:

Basic idea is to have 2 or more pending requests whose timeouts run out at
almost the same time.

req 1 timeout     ERP thread        req 2 timeout
----------------  ----------------  ---------------------------------------
zfcp_fsf_request_timeout_handler
fsf_req = from_timer(fsf_req, t, timer)
adapter = fsf_req->adapter
zfcp_qdio_siosl(adapter)
zfcp_erp_adapter_reopen(adapter,...)
                  zfcp_erp_strategy
                  ...
                  zfcp_fsf_req_dismiss_all
                  list_for_each_entry_safe
                    zfcp_fsf_req_complete 1
                    del_timer 1
                    zfcp_fsf_req_free 1
                    zfcp_fsf_req_complete 2
                                    zfcp_fsf_request_timeout_handler
                    del_timer 2
                                    fsf_req = from_timer(fsf_req, t, timer)
                    zfcp_fsf_req_free 2
                                    adapter = fsf_req->adapter
                                              ^^^^^^^ already freed

Link: https://lore.kernel.org/r/20200813152856.50088-1-maier@linux.ibm.com
Fixes: 75492a51 ("s390/scsi: Convert timers to use timer_setup()")
Cc: <stable@vger.kernel.org> #4.15+
Suggested-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/s390/scsi/zfcp_fsf.c

@@ -434,7 +434,7 @@ static void zfcp_fsf_req_complete(struct zfcp_fsf_req *req)
 		return;
 	}
 
-	del_timer(&req->timer);
+	del_timer_sync(&req->timer);
 	zfcp_fsf_protstatus_eval(req);
 	zfcp_fsf_fsfstatus_eval(req);
 	req->handler(req);
@@ -867,7 +867,7 @@ static int zfcp_fsf_req_send(struct zfcp_fsf_req *req)
 	req->qdio_req.qdio_outb_usage = atomic_read(&qdio->req_q_free);
 	req->issued = get_tod_clock();
 	if (zfcp_qdio_send(qdio, &req->qdio_req)) {
-		del_timer(&req->timer);
+		del_timer_sync(&req->timer);
 		/* lookup request again, list might have changed */
 		zfcp_reqlist_find_rm(adapter->req_list, req_id);
 		zfcp_erp_adapter_reopen(adapter, 0, "fsrs__1");