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