[Qemu-devel] possible denial of service via VNC

[Qemu-devel] possible denial of service via VNC

[Peter Lieven]

Hi,

while debugging a VNC issue I found this:

    case VNC_MSG_CLIENT_CUT_TEXT:
        if (len == 1)
            return 8;

        if (len == 8) {
            uint32_t dlen = read_u32(data, 4);
            if (dlen > 0)
                return 8 + dlen;
        }

        client_cut_text(vs, read_u32(data, 4), data + 8);
        break;

in protocol_client_msg().

Is this really a good idea? This allows for letting the vs->input buffer to grow
up to 2^32 + 8 byte which will possibly result in an out of memory condition.

Peter

Re: [Qemu-devel] possible denial of service via VNC

[Anthony Liguori]

On Sun, Jun 29, 2014 at 5:16 AM, Peter Lieven <pl@kamp.de> wrote:
> Hi,
>
> while debugging a VNC issue I found this:
>
>     case VNC_MSG_CLIENT_CUT_TEXT:
>         if (len == 1)
>             return 8;
>
>         if (len == 8) {
>             uint32_t dlen = read_u32(data, 4);
>             if (dlen > 0)
>                 return 8 + dlen;
>         }
>
>         client_cut_text(vs, read_u32(data, 4), data + 8);
>         break;
>
> in protocol_client_msg().
>
> Is this really a good idea? This allows for letting the vs->input buffer to grow
> up to 2^32 + 8 byte which will possibly result in an out of memory condition.

The spec allows cut operations of this size.  What would a reasonable limit be?

Regards,

Anthony Liguori

> PeterY
>
>

Re: [Qemu-devel] possible denial of service via VNC

[Gerd Hoffmann]

On So, 2014-06-29 at 14:16 +0200, Peter Lieven wrote:
> Hi,
> 
> while debugging a VNC issue I found this:
> 
>     case VNC_MSG_CLIENT_CUT_TEXT:
>         if (len == 1)
>             return 8;
> 
>         if (len == 8) {
>             uint32_t dlen = read_u32(data, 4);
>             if (dlen > 0)
>                 return 8 + dlen;
>         }
> 
>         client_cut_text(vs, read_u32(data, 4), data + 8);
>         break;
> 
> in protocol_client_msg().
> 
> Is this really a good idea? This allows for letting the vs->input buffer to grow
> up to 2^32 + 8 byte which will possibly result in an out of memory condition.

Applying a limit there looks reasonable to me.  Patches welcome.
As this is text only a megabyte should be more than enough for all
practical purposes.  Question is what to do when the limit is exceeded?
Disconnect?  Read & throw away?

cheers,
  Gerd

Re: [Qemu-devel] possible denial of service via VNC

[Peter Lieven]

On 30.06.2014 09:33, Gerd Hoffmann wrote:
> On So, 2014-06-29 at 14:16 +0200, Peter Lieven wrote:
>> Hi,
>>
>> while debugging a VNC issue I found this:
>>
>>      case VNC_MSG_CLIENT_CUT_TEXT:
>>          if (len == 1)
>>              return 8;
>>
>>          if (len == 8) {
>>              uint32_t dlen = read_u32(data, 4);
>>              if (dlen > 0)
>>                  return 8 + dlen;
>>          }
>>
>>          client_cut_text(vs, read_u32(data, 4), data + 8);
>>          break;
>>
>> in protocol_client_msg().
>>
>> Is this really a good idea? This allows for letting the vs->input buffer to grow
>> up to 2^32 + 8 byte which will possibly result in an out of memory condition.
> Applying a limit there looks reasonable to me.  Patches welcome.
> As this is text only a megabyte should be more than enough for all
> practical purposes.  Question is what to do when the limit is exceeded?
> Disconnect?  Read & throw away?

I would also think something in the order of megabytes should be fine.
I would vote for disconnect as soon as the limit specified is too big. Otherwise
we had to rewrite the whole receive logic which could introduce additional
bugs.

Peter

-- 

Mit freundlichen Grüßen

Peter Lieven

...........................................................

   KAMP Netzwerkdienste GmbH
   Vestische Str. 89-91 | 46117 Oberhausen
   Tel: +49 (0) 208.89 402-50 | Fax: +49 (0) 208.89 402-40
   pl@kamp.de | http://www.kamp.de

   Geschäftsführer: Heiner Lante | Michael Lante
   Amtsgericht Duisburg | HRB Nr. 12154
   USt-Id-Nr.: DE 120607556

...........................................................

Re: [Qemu-devel] possible denial of service via VNC

[Gerd Hoffmann]

  Hi,

> I would vote for disconnect as soon as the limit specified is too big. Otherwise
> we had to rewrite the whole receive logic which could introduce additional
> bugs.

Sounds sensible.

cheers,
  Gerd

Re: [Qemu-devel] possible denial of service via VNC

[Peter Lieven]

On 30.06.2014 09:46, Gerd Hoffmann wrote:
>    Hi,
>
>> I would vote for disconnect as soon as the limit specified is too big. Otherwise
>> we had to rewrite the whole receive logic which could introduce additional
>> bugs.
> Sounds sensible.

Especially since client_cut_text is currently a NOP.

Peter