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Re: [RFC PATCH 00/12] hw: Forbid DMA write accesses to MMIO regions

From: Edgar E. Iglesias
Subject: Re: [RFC PATCH 00/12] hw: Forbid DMA write accesses to MMIO regions
Date: Thu, 3 Sep 2020 16:24:10 +0200

On Thu, Sep 03, 2020 at 02:58:19PM +0100, Peter Maydell wrote:
> On Thu, 3 Sep 2020 at 14:37, Laszlo Ersek <lersek@redhat.com> wrote:
> > Peter mentions an approach at the end of
> > <https://bugs.launchpad.net/qemu/+bug/1886362/comments/5> that I believe
> > to understand, but -- according to him -- it seems too much work.
> It also would only be effective for MMIO, not for qemu_irq lines...
> > I don't think such chains work unto arbitrary depths on physical
> > hardware either.
> Real hardware by and large doesn't get designed with this kind
> of DMA-to-self as a consideration either, but unfortunately it's
> not really very useful as a model to base QEMU's behaviour on:
>  (1) real hardware is usually massively parallel, so the logic
>   that handles incoming MMIO is decoupled anyway from logic
>   that does outgoing DMA. (Arguably the "do all DMA in a
>   bottom-half" idea is kind of following the hardware design.)
>   Similarly simple "raise this outbound signal" logic just
>   works as an instantaneous action that causes the device on
>   the other end to change its state/do something parallel,
>   whereas for QEMU we need to actually call some code in the
>   device on the other end and so we serialize this stuff,
>   sandwiching a bit of "device B code" in the middle of a
>   run of "device A code". So a lot more of this stuff "just
>   happens to work" on h/w than we get with QEMU.
>  (2) if software running on real h/w does do something silly with
>   programming a device to DMA to itself then the worst case is
>   generally that they manage to wedge that device (or the whole
>   machine, if you're really unlucky), in which case the response
>   is "don't do that then". There isn't the same "guest code
>   can escape the VM" security boundary that QEMU needs to guard
>   against [*].
> [*] I do wonder about hardware-device-passthrough setups; I
> don't think I would care to pass through an arbitrary device
> to an untrusted guest...

Hmm, I guess it would make sense to have a configurable option in KVM
to isolate passthrough devices so they only can DMA to guest RAM...


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