Re: [Qemu-devel] [PATCH] usb: hcd-ehci: add check to avoid an infinite loop

[Gerd Hoffmann <kraxel@redhat.com>]

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On Do, 2015-12-10 at 18:51 +0530, P J P wrote:
>    Hello Gerd,
> 
> An infinite loop issue was reported by Mr Qinghao Tang(CC'd), in the USB EHCI 
> emulator. In that, a malicious isochronous transfer descriptor(iTD) list could 
> unfold an infinite loop in the 'ehci_advance_state' routine, by always 
> setting 'again = 0 or 1'.
> 
> Please see below a proposed (tested)patch to fix this issue. Does it look 
> okay? Not sure if 'count=16' is good for an upper limit.

Can you test the attached patch please?  In case it doesn't fix the bug:
Can you forward the reproducer to me?

thanks,
  Gerd

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From 6887f2191807c2b3eb7b20a61ba5d63c3695a95d Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 14 Dec 2015 09:21:23 +0100
Subject: [PATCH] ehci: make idt processing more robust

Make ehci_process_itd return an error in case we didn't do any actual
iso transfer because we've found no active transaction.  That'll avoid
ehci happily run in circles forever if the guest builds a loop out of
idts.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/usb/hcd-ehci.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index 4e2161b..d07f228 100644
--- a/hw/usb/hcd-ehci.c
+++ b/hw/usb/hcd-ehci.c
@@ -1389,7 +1389,7 @@ static int ehci_process_itd(EHCIState *ehci,
 {
     USBDevice *dev;
     USBEndpoint *ep;
-    uint32_t i, len, pid, dir, devaddr, endp;
+    uint32_t i, len, pid, dir, devaddr, endp, xfers = 0;
     uint32_t pg, off, ptr1, ptr2, max, mult;
 
     ehci->periodic_sched_active = PERIODIC_ACTIVE;
@@ -1479,9 +1479,10 @@ static int ehci_process_itd(EHCIState *ehci,
                 ehci_raise_irq(ehci, USBSTS_INT);
             }
             itd->transact[i] &= ~ITD_XACT_ACTIVE;
+            xfers++;
         }
     }
-    return 0;
+    return xfers ? 0 : -1;
 }
 
 
-- 
1.8.3.1