s390x/css: handle cssid 255 correctly

The cssid 255 is reserved but still valid from an architectural
point of view. However, feeding a bogus schid of 0xffffffff into
the virtio hypercall will lead to a crash:

Stack trace of thread 138363:
        #0  0x00000000100d168c css_find_subch (qemu-system-s390x)
        #1  0x00000000100d3290 virtio_ccw_hcall_notify
        #2  0x00000000100cbf60 s390_virtio_hypercall
        #3  0x000000001010ff7a handle_hypercall
        #4  0x0000000010079ed4 kvm_cpu_exec (qemu-system-s390x)
        #5  0x00000000100609b4 qemu_kvm_cpu_thread_fn
        #6  0x000003ff8b887bb4 start_thread (libpthread.so.0)
        #7  0x000003ff8b78df0a thread_start (libc.so.6)

This is because the css array was only allocated for 0..254
instead of 0..255.

Let's fix this by bumping MAX_CSSID to 255 and fencing off the
reserved cssid of 255 during css image allocation.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/css.c

@@ -141,7 +141,8 @@ int map_indicator(AdapterInfo *adapter, IndAddr *indicator)
 int css_create_css_image(uint8_t cssid, bool default_image)
 {
     trace_css_new_image(cssid, default_image ? "(default)" : "");
-    if (cssid > MAX_CSSID) {
+    /* 255 is reserved */
+    if (cssid == 255) {
         return -EINVAL;
     }
     if (channel_subsys.css[cssid]) {
@@ -1267,7 +1268,7 @@ bool css_schid_final(int m, uint8_t cssid, uint8_t ssid, uint16_t schid)
     uint8_t real_cssid;
 
     real_cssid = (!m && (cssid == 0)) ? channel_subsys.default_cssid : cssid;
-    if (real_cssid > MAX_CSSID || ssid > MAX_SSID ||
+    if (ssid > MAX_SSID ||
         !channel_subsys.css[real_cssid] ||
         !channel_subsys.css[real_cssid]->sch_set[ssid]) {
         return true;
@@ -1282,9 +1283,6 @@ static int css_add_virtual_chpid(uint8_t cssid, uint8_t chpid, uint8_t type)
     CssImage *css;
 
     trace_css_chpid_add(cssid, chpid, type);
-    if (cssid > MAX_CSSID) {
-        return -EINVAL;
-    }
     css = channel_subsys.css[cssid];
     if (!css) {
         return -EINVAL;

include/hw/s390x/css.h

@@ -20,7 +20,7 @@
 #define MAX_DEVNO 65535
 #define MAX_SCHID 65535
 #define MAX_SSID 3
-#define MAX_CSSID 254 /* 255 is reserved */
+#define MAX_CSSID 255
 #define MAX_CHPID 255
 
 #define MAX_CIWS 62