diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 29ba39f79e..3a67b67053 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1078,6 +1078,12 @@ static int css_interpret_ccw(SubchDev *sch, hwaddr ccw_addr,
             ret = -EINVAL;
             break;
         }
+        /* Limit the number of TICs in a given channel program */
+        if (sch->ccw_tic_cnt == 255) {
+            ret = -EINVAL;
+            break;
+        }
+        sch->ccw_tic_cnt++;
         sch->channel_prog = ccw.cda;
         ret = -EAGAIN;
         break;
@@ -1129,6 +1135,7 @@ static void sch_handle_start_func_virtual(SubchDev *sch)
         sch->ccw_fmt_1 = !!(orb->ctrl0 & ORB_CTRL0_MASK_FMT);
         schib->scsw.flags |= (sch->ccw_fmt_1) ? SCSW_FLAGS_MASK_FMT : 0;
         sch->ccw_no_data_cnt = 0;
+        sch->ccw_tic_cnt = 0;
         suspend_allowed = !!(orb->ctrl0 & ORB_CTRL0_MASK_SPND);
     } else {
         /* Start Function resumed via rsch */
diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
index 2d2cde7803..3c8cb16488 100644
--- a/hw/s390x/sclp.c
+++ b/hw/s390x/sclp.c
@@ -329,7 +329,8 @@ int sclp_service_call(S390CPU *cpu, uint64_t sccb, uint32_t code)
     /*
      * we want to work on a private copy of the sccb, to prevent guests
      * from playing dirty tricks by modifying the memory content after
-     * the host has checked the values
+     * the host has checked the values.
+     * Reuse the previously fetched header
      */
     work_sccb = g_malloc0(be16_to_cpu(header.length));
     ret = address_space_read(as, sccb, attrs,
@@ -337,6 +338,7 @@ int sclp_service_call(S390CPU *cpu, uint64_t sccb, uint32_t code)
     if (ret != MEMTX_OK) {
         return -PGM_ADDRESSING;
     }
+    work_sccb->h = header;
 
     if (!sclp_command_code_valid(code)) {
         work_sccb->h.response_code = cpu_to_be16(SCLP_RC_INVALID_SCLP_COMMAND);
diff --git a/hw/s390x/sclpcpi.c b/hw/s390x/sclpcpi.c
index 68fc1b809b..ec4bdf2350 100644
--- a/hw/s390x/sclpcpi.c
+++ b/hw/s390x/sclpcpi.c
@@ -97,6 +97,11 @@ static int write_event_data(SCLPEvent *event, EventBufferHeader *evt_buf_hdr)
                                              ebh);
     SCLPEventCPI *e = SCLP_EVENT_CPI(event);
 
+    /* Caller checks sccb length, buffer header checking is our duty */
+    if (be16_to_cpu(evt_buf_hdr->length) != sizeof(ControlProgramIdMsg)) {
+        return SCLP_RC_INCONSISTENT_LENGTHS;
+    }
+
     ascii_put(e->system_type, (char *)cpim->data.system_type,
               sizeof(cpim->data.system_type));
     ascii_put(e->system_name, (char *)cpim->data.system_name,
diff --git a/include/hw/s390x/css.h b/include/hw/s390x/css.h
index d3326237c9..79b41f305e 100644
--- a/include/hw/s390x/css.h
+++ b/include/hw/s390x/css.h
@@ -132,6 +132,7 @@ struct SubchDev {
     bool ccw_fmt_1;
     bool thinint_active;
     uint8_t ccw_no_data_cnt;
+    uint8_t ccw_tic_cnt;
     uint16_t migrated_schid; /* used for mismatch detection */
     CcwDataStream cds;
     /* transport-provided data: */