[Xen-devel] Re: [Qemu-devel] Recursion in cpu_physical_memory_rw

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

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.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
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