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[1/2] ehci: apply limit to itd/sidt descriptors

Message ID 1460971643-1499-1-git-send-email-kraxel@redhat.com
State New, archived
Headers show

Commit Message

Gerd Hoffmann April 18, 2016, 9:27 a.m. UTC
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(+)

Comments

P J P April 18, 2016, 11:52 a.m. UTC | #1
+-- On Mon, 18 Apr 2016, Gerd Hoffmann wrote --+
| 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.

  idt -> iTD
  sidt -> siTD
  Unfortualy -> Unfortunately
  reportly -> reportedly
  freebsd -> FreeBSD

Perhaps it'll help to add "Fixes: 156a2e4(CVE-2015-8558)" to the commit log? 
(just a thought)

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F
diff mbox

Patch

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);
 }