Re: [Xen-devel] [PATCH 3/4] x86/vmx: Fix security issue when a guest balloons out the #VE info page

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 2/20/19 12:18 AM, Andrew Cooper wrote:
> The logic in altp2m_vcpu_{en,dis}able_ve() and vmx_vcpu_update_vmfunc_ve() is
> dangerous.  After #VE has been set up, the guest can balloon out and free the
> nominated GFN, after which the processor may write to it.  Also, the unlocked
> GFN query means the MFN is stale by the time it is used.  Alternatively, a
> guest can race two disable calls to cause one VMCS to still reference the
> nominated GFN after the tracking information was dropped.
>
> Rework the logic from scratch to make it safe.
>
> Hold an extra page reference on the underlying frame, to account for the
> VMCS's reference.  This means that if the GFN gets ballooned out, it isn't
> freed back to Xen until #VE is disabled, and the VMCS no longer refers to the
> page.
>
> A consequence of this is that arch_vcpu_unmap_resources() now needs to call
> altp2m_vcpu_disable_ve() to drop the reference during domain_kill(), to allow
> all of the memory to be freed.
>
> For domains using altp2m, we expect a single enable call and no disable for
> the remaining lifetime of the domain.  However, to avoid problems with
> concurrent calls, use cmpxchg() to locklessly maintain safety.
>
> This doesn't have an XSA because altp2m is not yet a security-supported
> feature.
>
> Signed-off-by: Andrew Cooper<andrew.cooper3@xxxxxxxxxx>
Tested-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>


Thanks,
Razvan

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