ehci: apply limit to iTD/sidt descriptors

Commit "156a2e4 ehci: make idt processing more robust" tries to avoid a
DoS by the guest (create a circular iTD queue and let qemu ehci
emulation run in circles forever).  Unfortunately this has two problems:
First it misses the case of siTDs, and second it reportedly breaks
FreeBSD.

So lets go for a different approach: just count the number of iTDs and
siTDs we have seen per frame and apply a limit.  That should really
catch all cases now.

Reported-by: 杜少博 <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>

hw/usb/hcd-ehci.c

@@ -2011,6 +2011,7 @@ static int ehci_state_writeback(EHCIQueue *q)
 static void ehci_advance_state(EHCIState *ehci, int async)
 {
     EHCIQueue *q = NULL;
+    int itd_count = 0;
     int again;
 
     do {
@@ -2035,10 +2036,12 @@ static void ehci_advance_state(EHCIState *ehci, int async)
 
         case EST_FETCHITD:
             again = ehci_state_fetchitd(ehci, async);
+            itd_count++;
             break;
 
         case EST_FETCHSITD:
             again = ehci_state_fetchsitd(ehci, async);
+            itd_count++;
             break;
 
         case EST_ADVANCEQUEUE:
@@ -2087,7 +2090,8 @@ static void ehci_advance_state(EHCIState *ehci, int async)
             break;
         }
 
-        if (again < 0) {
+        if (again < 0 || itd_count > 16) {
+            /* TODO: notify guest (raise HSE irq?) */
             fprintf(stderr, "processing error - resetting ehci HC\n");
             ehci_reset(ehci);
             again = 0;