On 16 Oct 2020, at 22:01, Jann Horn wrote:
On Sat, Oct 17, 2020 at 6:34 AM Colm MacCarthaigh
<colmmacc@xxxxxxxxxx> wrote:
For user-space, even a single bit would do. We added
MADVISE_WIPEONFORK
so that userspace libraries can detect fork()/clone() robustly, for
the
same reasons. It just wipes a page as the indicator, which is
effectively a single-bit signal, and it works well. On the user-space
side of this, I’m keen to find a solution like that that we can use
fairly easily inside of portable libraries and applications. The
“have
I forked” checks do end up in hot paths, so it’s nice if they can
be
CPU cache friendly. Comparing a whole 128-bit value wouldn’t be my
favorite.
I'm pretty sure a single bit is not enough if you want to have a
single page, shared across the entire system, that stores the VM
forking state; you need a counter for that.
You’re right. WIPEONFORK is more like a single-bit per use. If it’s
something system wide then a counter is better.
So the RNG state after mixing in the new VM Generation ID would
contain 128 bits of secret entropy not known to anyone else, including
people with access to the VM image.
Now, 128 bits of cryptographically random data aren't _optimal_; I
think something on the order of 256 bits would be nicer from a
theoretical standpoint. But in practice I think we'll be good with the
128 bits we're getting (since the number of users who fork a VM image
is probably not going to be so large that worst-case collision
probabilities matter).
This reminds me on key/IV usage limits for AES encryption, where the
same birthday bounds apply, and even though 256-bits would be better, we
routinely make 128-bit birthday bounds work for massively scalable
systems.
The kernel would need to use the change as a trigger to
measure some entropy (e.g. interrupts and RDRAND, or whatever). Our
just
define the machine contract as “this has to be unique random data
and
if it’s not unique, or if it’s pubic, you’re toast”.
As far as I can tell from Microsoft's spec, that is a guarantee we're
already getting.
Neat.
-
Colm
