Re: [PATCH v3] hwrng: virtio: clamp device-reported used.len at copy_data()

[Herbert Xu]

On Thu, Jun 11, 2026 at 03:58:17AM -0400, Michael S. Tsirkin wrote:
> On Thu, Jun 11, 2026 at 03:46:58PM +0800, Herbert Xu wrote:
> > On Thu, Jun 11, 2026 at 03:30:14AM -0400, Michael S. Tsirkin wrote:
> > > On Thu, Jun 11, 2026 at 12:43:09PM +0800, Herbert Xu wrote:
> > > > On Sun, May 31, 2026 at 10:22:51AM -0400, Michael Bommarito wrote:
> > > > >
> > > > > +	size = min_t(unsigned int, size, avail - vi->data_idx);
> > > > > +	idx = array_index_nospec(vi->data_idx, sizeof(vi->data));
> > > > > +	memcpy(buf, vi->data + idx, size);
> > > 
> > > All the "malicious device" things are confusing. Spectre things -
> > > doubly so.
> > > 
> > > So if an access is speculated then CPU might speculate feeding a kernel
> > > secret into RNG. And then the speculated RNG value maybe can be also
> > > speculatively be used by some kernel code as an index
> > > to trigger a cache access, finally leaking the secret?
> > > 
> > > Maybe?
> > 
> > The way Spectre works is if you have an actual instruction using
> > idx directly.  I don't see how that translates to memcpy.
> 
> I am not sure it has to be direct:
> 
> if (malicious_idx > SIZE)
> 	return;
> src += malicious_idx;

Wait but vi->data_idx isn't even under the hypervisor's control.

It's an index maintained by our own driver.  So how can it be
malicious?

Cheers,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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