Re: propagating vmgenid outward and upward

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 04:36:49PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
> 
> On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote:
> > 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.
> 
> Because that 16 byte read of vmgenid is not atomic. Let's say you read
> the first 8 bytes, and then the VM is forked.

But at this point when VM was forked plaintext key and nonce are all in
buffer, and you previously indicated a fork at this point is harmless.
You wrote "If it changes _after_ that point of check ... it doesn't
matter:"

> In the forked VM, the next
> 8 bytes are the same as last time, but the first 8 bytes, which you
> already read, have changed. In that case, your != becomes a ==, and the
> test fails.

Yes I'm aware what an atomic read is. If the read is not atomic
a part of value can change ;)

-- 
MST