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

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.


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