[Xen-devel] Is: Make XENVER_* use XSM, seperate the different ops in smaller security domains. Was:Re: [PATCH v1 5/5] xsplice: Use ld-embedded build-ids

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

. snip..
> >>>> The build id of the current running hypervisor should belong in the
> >>>> xeninfo hypercall.  It is not specific to xsplice.
> >>> However in the previous reviews it was pointed out that it should only be 
> >>> accessible to dom0.
> >>>
> >>> Or to any domains as long as the XSM allows (and is turned on) - so not 
> >>> the default dummy one.
> >>>
> >>> That is a bit of 'if' extra complexity which I am not sure is worth it?
> >> DomU can already read the build information such as changeset, compile
> >> time, etc.  Build-id is no more special or revealing.
> > I would take this as an argument *against* giving DomU access to those
> > pieces of information in details and not as an argument for
> > *additionally* giving it access to build-id.
> >
> > With build-id we have the chance to shape a not-yet-established API and
> > I would like to follow the Principle of least privilege wherever it
> > makes sense.
> >
> > To reach a similar security level with the existing API, it might make
> > sense to limit DomU access to compile date, compile time, compiled by,
> > compiled domain, compiler version and command line details, xen extra
> > version, and xen changeset.  Basically anything that might help DomUs to
> > uniquely identify a Xen build.
> >
> > The approach can not directly drop access to those fields, as that would
> > break an existing API, but it could restrict the detail level handed out
> > to DomU.
> 
> These are all valid arguments to be made, but please lets fix the issue
> properly rather than hacking build-id on the side of an unrelated component.
> 
> From my point of view, the correct course of action is this:
> 
> * Split the current XSM model to contain separate attributes for general
> and privileged information.
> ** For current compatibility, all existing XENVER_* subops fall into general
> * Apply an XSM check at the very start of the hypercall.
> * Extend do_xen_version() to take 3 parameters.  (It is curious that it
> didn't take a length parameter before)
> ** This is still ABI compatible, as existing subops simply ignore the
> parameter.

Or we can just use 1024 bytes space the XENVER_* use.

> * Introduce new XENVER_build_id subop which is documented to require the
> 3-parameter version of the hypercall.
> ** This subop falls into straight into privileged information.
> 
> This will introduce build-id in its correct location, with appropriate
> restrictions.
> 
> Moving forwards, we really should have an audit of the existing XENVER_*
> subops.  Some are clearly safe/required for domU to read, but some such
> as XENVER_commandline have no business being accessible.  A separate
> argument, from the repeatable build point of view, says that compilation
> information isn't useful at all.
> 
> Depending on how we wish to fix the issue, we could either do a blanket
> move of the subops into the privileged XSM catagory, or introduce a 3rd
> "legacy privileged" category to allow the admin to control access on a
> per-vm basis.

CC-ing Daniel. Changing title.
> 
> ~Andrew

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