Re: [Xen-devel] [PATCH v5] altp2m: Allow specifying external-only use-case

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 03/22/2017 02:07 PM, Tamas K Lengyel wrote:
> Currently setting altp2mhvm=1 in the domain configuration allows access to the
> altp2m interface for both in-guest and external privileged tools. This poses
> a problem for use-cases where only external access should be allowed, requiring
> the user to compile Xen with XSM enabled to be able to appropriately restrict
> access.
>
> In this patch we deprecate the altp2mhvm domain configuration option and
> introduce the altp2m option, which allows specifying if by default the altp2m
> interface should be external-only. The information is stored in
> HVM_PARAM_ALTP2M which we now define with specific XEN_ALTP2M_* modes.
> If external mode is selected, the XSM check is shifted to use XSM_DM_PRIV
> type check, thus restricting access to the interface by the guest itself. Note
> that we keep the default XSM policy untouched. Users of XSM who wish to enforce
> external mode for altp2m can do so by adjusting their XSM policy directly,
> as this domain config option does not override an active XSM policy.
>
> Also, as part of this patch we adjust the hvmop handler to require
> HVM_PARAM_ALTP2M to be of a type other then disabled for all ops. This has been
> previously only required for get/set altp2m domain state, all other options
> were gated on altp2m_enabled. Since altp2m_enabled only gets set during set
> altp2m domain state, this change introduces no new requirements to the other
> ops but makes it more clear that it is required for all ops.
>
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
> Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
I think the XSM-enabled case using the default types should have the same
flexibility as the XSM-disabled case.  I agree that it is useful to be able
to restrict the p2m features based on policy, and I don't think that it's
useful to expand the number of XSM permissions here.  In that case, the best
way to proceed would be to require that both the domain configuration and
XSM policy must allow the action (similar to how SELinux file controls and
UNIX permissions interact).  Currently, enabling XSM effectively forces the
value of this setting to "mixed", and "limited" is impossible to use with XSM.

--
Daniel De Graaf
National Security Agency

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel