Re: [Qemu-devel] Recursion in cpu_physical_memory_rw

[Anthony Liguori <aliguori@us.ibm.com>]

Herbert Xu wrote:
> On Wed, Nov 15, 2006 at 12:57:24AM +0000, Paul Brook wrote:
>> It isn't always system memory. Some DMA controllers deliberately write to 
>> device FIFOs. There are also several devices which map areas of onboard RAM. 
>> At minimum you need to make those to use RAM mappings rather than MMIO.
> 
> I'm not suggesting that we change all existing users of cpu_physical_*
> to a new interface that only accessed RAM.  However, for cases where it
> is obvious that only system RAM is intended (e.g., rtl8139), it makes
> sense to bypass MMIO handlers.
> 
>> If a device is recursively writing to itself I'd take this as sign that the 
>> guest OS is already pretty screwed. I'm not sure what happens in this 
>> situation on real hardware, but I wouldn't be surprised if it caused similar 
>> effects by flooding the bus.
> 
> The scenario here is a compromised guest attempting to harm a host such
> as Xen.

The only "harm" done to a host is that the process will take as much CPU 
as it can get.  This is really only a problem in Xen because the device 
model is in Domain-0.  Once the device model is in a different domain, 
it doesn't matter anymore as the normal scheduler parameters can be used 
to ensure that no other hosts are harmed.

Regards,

Anthony Liguori


> Cheers,