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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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