Re: [Xen-devel] [PATCH 13/13] x86/domctl: Implement XEN_DOMCTL_set_cpumsr_policy

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jul 03, 2018 at 09:55:26PM +0100, Andrew Cooper wrote:
> From: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> 
> This hypercall allows the toolstack to present one combined CPUID and MSR
> policy for a domain, which can be audited in one go by Xen, which is necessary
> for correctness of the auditing.
> 
> A stub x86_policies_are_compatible() function is introduced, although at
> present it will always fail the hypercall.
> 
> The hypercall ABI allows for update of individual CPUID or MSR entries, so
> begins by duplicating the existing policy (for which a helper is introduced),
> merging the toolstack data, then checking compatibility of the result.
> 
> The system PV/HVM max policy is used for the compatiblity check.
> 
> Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> CC: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> 
> One awkard corner case is re-deserialising of the vcpu msrs.  The correct fix
> would be to allocate a buffer, copy the MSRs list, then deserialise from that,
> but trips the bounds checks in the copy_from_guest() helpers.  The compat XLAT
> are would work, but would require that we allocate it even for 64bit PV
> guests.

I'm not sure I follow this. The issue isn't obvious from looking at the
code.

> ---
> +    /* Merge the (now audited) vCPU MSRs into every other msr_vcpu_policy. */
> +    for ( ; v; v = v->next_in_list )
> +    {
> +        /* XXX - Figure out how to avoid a TOCTOU race here.  XLAT area? */

What is the TOCTOU race here? 

Wei.

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