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Re: [Qemu-devel] Recursion in cpu_physical_memory_rw

From: Anthony Liguori
Subject: Re: [Qemu-devel] Recursion in cpu_physical_memory_rw
Date: Wed, 15 Nov 2006 19:04:01 -0000
User-agent: Thunderbird (X11/20060918)

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.


Anthony Liguori


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