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

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;
  2159     if ( altp2m_active(d) )
  2160         p2m = p2m_get_altp2m(v);
  2161     if ( !p2m )
  2162         p2m = p2m_get_hostp2m(d);
  2164     ept = &p2m->ept;
  2165     ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
  2167     vmx_vmcs_enter(v);
  2169     __vmwrite(EPT_POINTER, ept->eptp);
  2171     if ( v->arch.hvm.vmx.secondary_exec_control &
  2173         __vmwrite(EPTP_INDEX, vcpu_altp2m(v).p2midx);
  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.


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