Re: [RFC PATCH 00/13] Core scheduling v5

Joel Fernandes
2020-04-18T02:26:53Z

Hi Peter,

On Fri, Apr 17, 2020 at 01:12:55PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 15, 2020 at 12:32:20PM -0400, Joel Fernandes wrote:
> > On Tue, Apr 14, 2020 at 04:21:52PM +0200, Peter Zijlstra wrote:
> > > On Wed, Mar 04, 2020 at 04:59:50PM +0000, vpillai wrote:
> > > > TODO
> > > > ----
> > > > - Work on merging patches that are ready to be merged
> > > > - Decide on the API for exposing the feature to userland
> > > > - Experiment with adding synchronization points in VMEXIT to mitigate
> > > > the VM-to-host-kernel leaking
> > >
> > > VMEXIT is too late, you need to hook irq_enter(), which is what makes
> > > the whole thing so horrible.
> >
> > We came up with a patch to do this as well. Currently testing it more and it
> > looks clean, will share it soon.
>
> Thomas said we actually first do VMEXIT, and then enable interrupts. So
> the VMEXIT thing should actually work, and that is indeed much saner
> than sticking it in irq_enter().
>
> It does however put yet more nails in the out-of-tree hypervisors.

Just to clarify what we're talking about here. The condition we are trying to
protect against:

1. VM is malicious.
2. Sibling of VM is entering an interrupt on the host.
3. When we enter the interrupt, we send an IPI to force the VM into waiting.
4. The VM on the sibling enters VMEXIT.

In step 4, we have to synchronize. Is this the scenario we are discussing?

> > > > - Investigate the source of the overhead even when no tasks are tagged:
> > > > https://lkml.org/lkml/2019/10/29/242
> > >
> > > - explain why we're all still doing this ....
> > >
> > > Seriously, what actual problems does it solve? The patch-set still isn't
> > > L1TF complete and afaict it does exactly nothing for MDS.
> >
> > The L1TF incompleteness is because of cross-HT attack from Guest vCPU
> > attacker to an interrupt/softirq executing on the other sibling correct? The
> > IRQ enter pausing the other sibling should fix that (which we will share in
> > a future series revision after adequate testing).
>
> Correct, the vCPU still running can glean host (kernel) state from the
> sibling handling the interrupt in the host kernel.

Right. This is what we're handling.

> > > Like I've written many times now, back when the world was simpler and
> > > all we had to worry about was L1TF, core-scheduling made some sense, but
> > > how does it make sense today?
> >
> > For ChromeOS we're planning to tag each and every task seperately except for
> > trusted processes, so we are isolating untrusted tasks even from each other.
> >
> > Sorry if this sounds like pushing my usecase, but we do get parallelism
> > advantage for the trusted tasks while still solving all security issues (for
> > ChromeOS). I agree that cross-HT user <-> kernel MDS is still an issue if
> > untrusted (tagged) tasks execute together on same core, but we are not
> > planning to do that on our setup at least.
>
> That doesn't completely solve things I think. Even if you run all
> untrusted tasks as core exclusive, you still have a problem of them vs
> interrupts on the other sibling.
>
> You need to somehow arrange all interrupts to the core happen on the
> same sibling that runs your untrusted task, such that the VERW on
> return-to-userspace works as intended.
>
> I suppose you can try and play funny games with interrupt routing tied
> to the force-idle state, but I'm dreading what that'll look like. Or
> were you going to handle this from your irq_enter() thing too?

Yes, even when host interrupt is on one sibling and the untrusted host
process is on the other sibling, we would be handling it the same way we
handle it for host interrupts vs untrusted guests. Perhaps we could optimize
pausing of the guest. But Vineeth tested and found that the same code that
pauses hosts also works for guests.

> Can someone go write up a document that very clearly states all the
> problems and clearly explains how to use this feature?

A document would make sense on how to use the feature. Maybe we can add it as
a documentation patch to the series?

Basically, from my notes the following are the problems:

Core-scheduling will help with cross-HT MDS and L1TF attacks. The following
are the scenarios (borrowed from an email from Thomas -- thanks!):

HT1 (attack) HT2 (victim)

A idle -> user space user space -> idle

B idle -> user space guest -> idle

C idle -> guest user space -> idle

D idle -> guest guest -> idle

All of them suffer from MDS. #C and #D suffer from L1TF.

All of these scenarios result in the victim getting idled to prevent any
leakage. However, this does not address the case where victim is an
interrupt handler or softirq. So we need to either route interrupts to run on
the attacker CPU, or have the irq_enter() send IPIs to pause the sibling
which is what we prototyped. Another approach to solve this is to force
interrupts into threaded mode as Thomas suggested.

The problematic usecase then left (if we ignore IRQ troubles), is MDS issues
between user <-> kernel simultaneous execution on both siblings. This does
not become an issue on ChromeOS where everything untrusted has its own tag.
For Vineeth's VM workloads also this isn't a problem from my discussions with
him, as he mentioned hyperthreads are both running guests and 2 vCPU threads
that belong to different VMs will not execute on the same core (though I'm
not sure whether hypercalls from a vCPU when the sibling is running another
vCPU of the same VM, is a concern here).

Any other case that needs to be considered?

thanks,

- Joel