Re: [Xen-devel] [PATCH for-4.12] x86/altp2m: fix HVMOP_altp2m_set_domain_state race

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 2/8/19 1:13 PM, Razvan Cojocaru wrote:
> On 2/8/19 12:51 PM, Jan Beulich wrote:
> > > > > On 08.02.19 at 10:56, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > > HVMOP_altp2m_set_domain_state does not domain_pause(), presumably
> > > on purpose (as it was originally supposed to cater to a in-guest
> > > agent, and a domain pausing itself is not a good idea).
> > >
> > > This can lead to domain crashes in the vmx_vmexit_handler() code
> > > that checks if the guest has the ability to switch EPTP without an
> > > exit. That code can __vmread() the host p2m's EPT_POINTER
> > > (before HVMOP_altp2m_set_domain_state "for_each_vcpu()" has a
> > > chance to run altp2m_vcpu_initialise(), but after
> > > d->arch.altp2m_active is set).
> > >
> > > While the in-guest scenario continues to pose problems, this
> > > patch fixes the "external" case.
> > IOW you're papering over the problem rather than fixing it. Why
> > does altp2m_active get set to true before actually having set up
> > everything? Shouldn't it get cleared early, but set late?
> Well, yes, that would have been my second attempt: set the "altp2m 
> enabled" bool after the init, and before the uninit and no longer 
> domain_pause() explicitly; however I thought that was a brittle 
> solution, relying on comments / programmer attention to the code 
> sequence rather than taking a proper lock.
> I'll test that scenario then and return with the results / possibly 
> another patch.
Actually, your suggestion does not work, because the way the code has 
been designed, altp2m_vcpu_initialise() calls altp2m_vcpu_update_p2m(), 
which does the proper work that's interesting to us here, like this:
  2153 static void vmx_vcpu_update_eptp(struct vcpu *v)
  2154 {
  2155     struct domain *d = v->domain;
  2156     struct p2m_domain *p2m = NULL;
  2157     struct ept_data *ept;
  2158
  2159     if ( altp2m_active(d) )
  2160         p2m = p2m_get_altp2m(v);
  2161     if ( !p2m )
  2162         p2m = p2m_get_hostp2m(d);
  2163
  2164     ept = &p2m->ept;
  2165     ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
  2166
  2167     vmx_vmcs_enter(v);
  2168
  2169     __vmwrite(EPT_POINTER, ept->eptp);
  2170
  2171     if ( v->arch.hvm.vmx.secondary_exec_control &
  2172          SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS )
  2173         __vmwrite(EPTP_INDEX, vcpu_altp2m(v).p2midx);
  2174
  2175     vmx_vmcs_exit(v);
  2176 }

So please note that on line 2159 it checks if altp2m is active, and only 
then does it do the right thing. So setting the d->arch.altp2m_active 
bool _after_ calling altp2m_vcpu_initialise() will fail to work 
correctly - turning this into a chicken-and-egg problem, or perhaps more 
interestingly, another discussion about whether in-guest-only altp2m 
agents make any sense fundamentally.
I think that, sadly, the best we can do at this time is the patch I've sent.


Thanks,
Razvan

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