Re: propagating vmgenid outward and upward

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 02:42:37AM -0500, Michael S. Tsirkin wrote:
> On Tue, Mar 01, 2022 at 07:37:06PM +0100, Jason A. Donenfeld wrote:
> > Hi Michael,
> > 
> > On Tue, Mar 1, 2022 at 6:17 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > Hmm okay, so it's a performance optimization... some batching then? Do
> > > you really need to worry about every packet? Every 64 packets not
> > > enough?  Packets are after all queued at NICs etc, and VM fork can
> > > happen after they leave wireguard ...
> > 
> > Unfortunately, yes, this is an "every packet" sort of thing -- if the
> > race is to be avoided in a meaningful way. It's really extra bad:
> > ChaCha20 and AES-CTR work by xoring a secret stream of bytes with
> > plaintext to produce a ciphertext. If you use that same secret stream
> > and xor it with a second plaintext and transmit that too, an attacker
> > can combine the two different ciphertexts to learn things about the
> > original plaintext.
> 
> So what about the point about packets queued then? You don't fish
> packets out of qdisc queues, do you?

Oh pls ignore it, I think I got it. Resending same packet is not
a problem, producing a new one is.

> > But, anyway, it seems like the race is here to stay given what we have
> > _currently_ available with the virtual hardware. That's why I'm
> > focused on trying to get something going that's the least bad with
> > what we've currently got, which is racy by design. How vitally
> > important is it to have something that doesn't race in the far future?
> > I don't know, really. It seems plausible that that ACPI notifier
> > triggers so early that nothing else really even has a chance, so the
> > race concern is purely theoretical. But I haven't tried to measure
> > that so I'm not sure.
> > 
> > Jason


So how about measuring the performance impact of reading the 16 byte
vmgenid then? This could be a performance option, too - some people
might want extra security, some might not care.  And I feel if linux
DTRT and reads the 16 bytes then hypervisor vendors will be motivated to
improve and add a 4 byte unique one. As long as linux is interrupt
driven there's no motivation for change.

-- 
MST