Re: [RFC PATCH v3 00/16] Core scheduling v3

[Phil Auld]

On Wed, Aug 28, 2019 at 06:01:14PM +0200 Peter Zijlstra wrote:
> On Wed, Aug 28, 2019 at 11:30:34AM -0400, Phil Auld wrote:
> > On Tue, Aug 27, 2019 at 11:50:35PM +0200 Peter Zijlstra wrote:
> 
> > > And given MDS, I'm still not entirely convinced it all makes sense. If
> > > it were just L1TF, then yes, but now...
> > 
> > I was thinking MDS is really the reason for this. L1TF has mitigations but
> > the only current mitigation for MDS for smt is ... nosmt. 
> 
> L1TF has no known mitigation that is SMT safe. The moment you have
> something in your L1, the other sibling can read it using L1TF.
> 
> The nice thing about L1TF is that only (malicious) guests can exploit
> it, and therefore the synchronizatin context is VMM. And it so happens
> that VMEXITs are 'rare' (and already expensive and thus lots of effort
> has already gone into avoiding them).
> 
> If you don't use VMs, you're good and SMT is not a problem.
> 
> If you do use VMs (and do/can not trust them), _then_ you need
> core-scheduling; and in that case, the implementation under discussion
> misses things like synchronization on VMEXITs due to interrupts and
> things like that.
> 
> But under the assumption that VMs don't generate high scheduling rates,
> it can work.
> 
> > The current core scheduler implementation, I believe, still has (theoretical?) 
> > holes involving interrupts, once/if those are closed it may be even less 
> > attractive.
> 
> No; so MDS leaks anything the other sibling (currently) does, this makes
> _any_ privilidge boundary a synchronization context.
> 
> Worse still, the exploit doesn't require a VM at all, any other task can
> get to it.
> 
> That means you get to sync the siblings on lovely things like system
> call entry and exit, along with VMM and anything else that one would
> consider a privilidge boundary. Now, system calls are not rare, they
> are really quite common in fact. Trying to sync up siblings at the rate
> of system calls is utter madness.
> 
> So under MDS, SMT is completely hosed. If you use VMs exclusively, then
> it _might_ work because a 'pure' host doesn't schedule that often
> (maybe, same assumption as for L1TF).
> 
> Now, there have been proposals of moving the privilidge boundary further
> into the kernel. Just like PTI exposes the entry stack and code to
> Meltdown, the thinking is, lets expose more. By moving the priv boundary
> the hope is that we can do lots of common system calls without having to
> sync up -- lots of details are 'pending'.


Thanks for clarifying. My understanding is (somewhat) less fuzzy now. :)

I think, though, that you were basically agreeing with me that the current 
core scheduler does not close the holes, or am I reading that wrong.


Cheers,
Phil

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