[PATCH v2] hw/usb/hcd-ohci: check for MPS=0 to avoid infinite loop

[Jenny Guanni Qu <qguanni@gmail.com>]

When a guest sets MaxPacketSize to 0 in an OHCI Endpoint Descriptor,
ohci_service_td() transfers 0 bytes per iteration. The Transfer
Descriptor never completes because CBP never advances toward BE,
causing ohci_service_ed_list() to loop indefinitely and hang QEMU.

Add a check for MPS==0 after extracting the field from ED flags.
If MPS is zero, call ohci_die() to reset the controller and return
an error, preventing the infinite loop.

Fixes: CVE-2026-3890
Reported-by: Jenny Guanni Qu <qguanni@gmail.com>
Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
---
 hw/usb/hcd-ohci.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c
index 1aeed9286f..6ed8046fc2 100644
--- a/hw/usb/hcd-ohci.c
+++ b/hw/usb/hcd-ohci.c
@@ -956,6 +956,17 @@ static int ohci_service_td(OHCIState *ohci, struct ohci_ed *ed)
         if (len && dir != OHCI_TD_DIR_IN) {
             /* The endpoint may not allow us to transfer it all now */
             pktlen = (ed->flags & OHCI_ED_MPS_MASK) >> OHCI_ED_MPS_SHIFT;
+            /*
+             * The OHCI spec does not say what to do if the guest hands us
+             * an endpoint descriptor which specifies a MaximumPacketSize
+             * of zero, which would mean we can never actually make forward
+             * progress transferring data to it. We choose to treat it as
+             * an error.
+             */
+            if (pktlen == 0) {
+                ohci_die(ohci);
+                return 1;
+            }
             if (pktlen > len) {
                 pktlen = len;
             }
-- 
2.34.1