linux-kernel - Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains

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Message-ID: <ZlAEjMIpSG8PoMnc@yzhao56-desk.sh.intel.com>
Date: Fri, 24 May 2024 11:07:56 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Alex Williamson <alex.williamson@...hat.com>, "Tian, Kevin"
	<kevin.tian@...el.com>, "Vetter, Daniel" <daniel.vetter@...el.com>,
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	<baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in
 non-coherent domains

On Wed, May 22, 2024 at 09:26:05AM -0300, Jason Gunthorpe wrote:
> On Wed, May 22, 2024 at 11:24:20AM +0800, Yan Zhao wrote:
> > On Tue, May 21, 2024 at 01:34:00PM -0300, Jason Gunthorpe wrote:
> > > On Tue, May 21, 2024 at 10:21:23AM -0600, Alex Williamson wrote:
> > > 
> > > > > Intel GPU weirdness should not leak into making other devices
> > > > > insecure/slow. If necessary Intel GPU only should get some variant
> > > > > override to keep no snoop working.
> > > > > 
> > > > > It would make alot of good sense if VFIO made the default to disable
> > > > > no-snoop via the config space.
> > > > 
> > > > We can certainly virtualize the config space no-snoop enable bit, but
> > > > I'm not sure what it actually accomplishes.  We'd then be relying on
> > > > the device to honor the bit and not have any backdoors to twiddle the
> > > > bit otherwise (where we know that GPUs often have multiple paths to get
> > > > to config space).
> > > 
> > > I'm OK with this. If devices are insecure then they need quirks in
> > > vfio to disclose their problems, we shouldn't punish everyone who
> > > followed the spec because of some bad actors.
> > Does that mean a malicous device that does not honor the bit could read
> > uninitialized host data?
> 
> Yes, but a malicious device could also just do DMA with the PF RID and
> break everything. VFIO substantially trusts the device already, I'm
> not sure trusting it to do no-snoop blocking is a big reach.
There are minor differences between the two trusts though.
With no-snoop, the page is possible to be critical data previously used by
kernel core.
But with a fake RID, a malicious device can at least access memory allowed
for other devices.