Re: [Qemu-devel] [CVE-2014-3615 PATCH v2 3/3] spice: make sure we don't overflow ssd->buf

[Laszlo Ersek]

On 09/05/14 10:58, Gerd Hoffmann wrote:
>   Hi,
> 
>> I can't track this back far enough. I'd feel safer if you checked that
>> the multiplication can't overflow even in uint64_t.
> 
> Effectively it comes from the emulated graphics hardware (anything in
> hw/display/*).  The gfx emulation must make sure that the framebuffer
> fits into the video memory, which in turn pretty much implies that we
> can't overflow uint64_t.  I think even uint32_t can't overflow with the
> gfx hardware we are emulating today.
> 
>> (5) Instead, you really need to make sure that the very first
>> multiplication fits in a signed int:
> 
> Makes sense.  I think it is easier to just multiply in 64bit, then check
> the result is small enougth (new patch attached).

Okay, if you can guarantee that the product fits in uint64_t, then such
a check would suffice.

New patch has not been attached though :)

> 
>>>  /* display listener callbacks */
>>> @@ -495,7 +503,7 @@ static void interface_get_init_info(QXLInstance *sin, 
>>> QXLDevInitInfo *info)
>>>      info->num_memslots = NUM_MEMSLOTS;
>>>      info->num_memslots_groups = NUM_MEMSLOTS_GROUPS;
>>>      info->internal_groupslot_id = 0;
>>> -    info->qxl_ram_size = ssd->bufsize;
>>> +    info->qxl_ram_size = 16 * 1024 * 1024;
>>>      info->n_surfaces = ssd->num_surfaces;
>>>  }
> 
> spice-server doesn't do anything with it, other than passing to
> spice-client.  Not fully sure what spice-client uses this for, maybe as
> some kind of hint for resource management.  Maybe not at all.
> 
> It surely doesn't matter at all for ssd->buf size.

Okay, I'll trust you on this.

Thanks
Laszlo