[PATCH] hw/usb/hcd-ohci: Make sure that ohci_service_ed_list() cannot loop forever

[Thomas Huth <thuth@redhat.com>]

From: Thomas Huth <thuth@redhat.com>

The inner while loop in ohci_service_ed_list() could theoretically
loop forever if a malicious guest prepares a set of bad descriptors.
Add a check to the loop to avoid this situation.

Reported-by: Feifan Qian <bea1e@proton.me>
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3781
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/usb/hcd-ohci.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c
index 40ebafb4dd9..c4a9741bd42 100644
--- a/hw/usb/hcd-ohci.c
+++ b/hw/usb/hcd-ohci.c
@@ -28,6 +28,7 @@
 #include "qemu/osdep.h"
 #include "hw/core/irq.h"
 #include "qapi/error.h"
+#include "qemu/log.h"
 #include "qemu/module.h"
 #include "qemu/timer.h"
 #include "hw/usb/usb.h"
@@ -1129,6 +1130,8 @@ static int ohci_service_ed_list(OHCIState *ohci, uint32_t head)
         return 0;
     }
     for (cur = head; cur && link_cnt++ < ED_LINK_LIMIT; cur = next_ed) {
+        unsigned int ed_cnt = 0;
+
         if (ohci_read_ed(ohci, cur, &ed)) {
             trace_usb_ohci_ed_read_error(cur);
             ohci_die(ohci);
@@ -1172,6 +1175,13 @@ static int ohci_service_ed_list(OHCIState *ohci, uint32_t head)
                     break;
                 }
             }
+
+            if (ed_cnt++ > ED_LINK_LIMIT) {
+                qemu_log_mask(LOG_GUEST_ERROR,
+                              "ohci: Too many endpoint descriptors in loop\n");
+                ohci_die(ohci);
+                return 0;
+            }
         }
 
         if (ohci_put_ed(ohci, cur, &ed)) {
-- 
2.55.0