Re: propagating vmgenid outward and upward

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 04:14:56PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
> 
> On Wed, Mar 2, 2022 at 3:46 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > I just don't see how "value changed while it was read" is so different
> > from "value changed one clock after it was read".  Since we don't detect
> > the latter I don't see why we should worry about the former.
> 
> The "barrier" is at the point where the plaintext has been chosen AND
> the nonce for a given keypair has been selected. So, if you have
> plaintext in a buffer, and a key in a buffer, and the nonce for that
> encryption in a buffer, and then after those are all selected, you
> check to see if the vmgenid has changed since the birth of that key,
> then you're all set. If it changes _after_ that point of check (your
> "one clock after"), it doesn't matter: you'll just be
> double-transmitting the same ciphertext, which is something that flaky
> wifi sometimes does _anyway_ (and attackers can do intentionally), so
> network protocols already are resilient to replay. This is the same
> case you asked about earlier, and then answered yourself, when you
> were wondering about reaching down into qdiscs.
> 
> Jason

So writing some code:

1:
        put plaintext in a buffer
        put a key in a buffer
        put the nonce for that encryption in a buffer

        if vm gen id != stored vm gen id
                stored vm gen id = vm gen id
                goto 1

I think this is race free, but I don't see why does it matter whether we
read gen id atomically or not.

-- 
MST