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

[Herbert Xu]

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.

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