[Qemu-devel] [PATCH 1/2] ehci: apply limit to itd/sidt descriptors

[Gerd Hoffmann <kraxel@redhat.com>]

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).  Unfortunaly this has two problems:
First it misses the case of sitds, and second it reportly 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.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/usb/hcd-ehci.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index 159f58d..923f110 100644
--- a/hw/usb/hcd-ehci.c
+++ b/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 idt_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);
+            idt_count++;
             break;
 
         case EST_FETCHSITD:
             again = ehci_state_fetchsitd(ehci, async);
+            idt_count++;
             break;
 
         case EST_ADVANCEQUEUE:
@@ -2092,6 +2095,11 @@ static void ehci_advance_state(EHCIState *ehci, int async)
             ehci_reset(ehci);
             again = 0;
         }
+
+        /* limit the amout of idts we are willing to process each frame */
+        if (idt_count > 16) {
+            again = 0;
+        }
     }
     while (again);
 }
-- 
1.8.3.1