Re: [Xen-devel] [PATCH V3] x86/altp2m: Fix crash with INVALID_ALTP2M EPTP index

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 06/27/2018 12:46 PM, Jan Beulich wrote:
>>>> On 26.06.18 at 16:21, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> When SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS is set,
>> vmx_vcpu_update_eptp() __vmwrites() EPTP_INDEX in
>> altp2m_vcpu_destroy(). This means that when disabling altp2m on a
>> domain after xc_altp2m_set_vcpu_enable_notify() has been
>> successfully called, EPTP_INDEX ends up being stored as
>> INVALID_ALTP2M. This makes it possible for vmx_vmexit_handler()
>> to __vmread() the stale value after a subsequent call to
>> xc_altp2m_set_vcpu_enable_notify(), and BUG_ON(idx >= MAX_ALTP2M).
> 
> I'm fine with the code change now, but I think this 3rd approach
> of addressing the issue needs the description to be changed.
> Already on v2 it wouldn't have become clear to me what the
> issue was from just reading the description. In particular you now
> want to point out why the change is correct / necessary also for
> the other invocation of altp2m_vcpu_update_vmfunc_ve(). It
> would also be helpful to have a statement on why other
> altp2m_vcpu_update_p2m() invocations don't need to be
> prefixed (now: replaced) by altp2m_vcpu_update_vmfunc_ve().
> In the end it might well be that folding the two hooks into one is
> the best course of action.

I'll do my best to make the description more readable. As for folding
the two hooks into one (I assume you mean having a single function, such
as, e.g. altp2m_vcpu_update_ve_and_p2m() and removing the other two), it
looks like vmx_vcpu_update_vmfunc_ve() does a few things that would be
unnnecessary (not optimal) in the general case. For example it calls
__vmwrite(VM_FUNCTION_CONTROL, VMX_VMFUNC_EPTP_SWITCHING);, which
shouldn't necessarily happen at the callsites of
altp2m_vcpu_update_p2m(v) in p2m.c (in p2m_switch_vcpu_altp2m_by_id()
and p2m_switch_domain_altp2m_by_id()). So from that point of view, it
may be worth to keep both altp2m_vcpu_update_p2m() and
altp2m_vcpu_update_vmfunc_ve() (the latter still always needing to call
the former to do its job properly).

It's possible that I've misunderstood your comment here though.


Thanks,
Razvan

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