Re: [PATCH v3 9/9] x86/P2M: relax permissions of PVH Dom0's MMIO entries

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Sep 23, 2021 at 01:32:52PM +0200, Jan Beulich wrote:
> On 23.09.2021 13:10, Roger Pau Monné wrote:
> > On Tue, Sep 21, 2021 at 09:21:11AM +0200, Jan Beulich wrote:
> >> To become independent of the sequence of mapping operations, permit
> >> "access" to accumulate for Dom0, noting that there's not going to be an
> >> introspection agent for it which this might interfere with. While e.g.
> >> ideally only ROM regions would get mapped with X set, getting there is
> >> quite a bit of work. Plus the use of p2m_access_* here is abusive in the
> >> first place.
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >> ---
> >> v3: Move last in series, for being controversial.
> >> v2: Split off from original patch. Accumulate all of R, W, and X.
> >>
> >> --- a/xen/arch/x86/mm/p2m.c
> >> +++ b/xen/arch/x86/mm/p2m.c
> >> @@ -1319,6 +1319,18 @@ static int set_typed_p2m_entry(struct do
> >>              return -EPERM;
> >>          }
> >>  
> >> +        /*
> >> +         * Gross bodge, to go away again rather sooner than later:
> >> +         *
> >> +         * For MMIO allow access permissions to accumulate, but only for Dom0.
> >> +         * Since set_identity_p2m_entry() and set_mmio_p2m_entry() differ in
> >> +         * the way they specify "access", this will allow the ultimate result
> >> +         * to be independent of the sequence of operations.
> > 
> > Wouldn't it be better to 'fix' those operations so that they can work
> > together?
> 
> Yes, but then we should do this properly by removing all abuse of
> p2m_access_t.

I'm not sure I follow what that abuse is.

> > It's my understanding that set_identity_p2m_entry is the one that has
> > strong requirements regarding the access permissions, as on AMD ACPI
> > tables can specify how should regions be mapped.
> > 
> > A possible solution might be to make set_mmio_p2m_entry more tolerant
> > to how present mappings are handled. For once that function doesn't
> > let callers specify access permissions, so I would consider that if a
> > mapping is present on the gfn and it already points to the requested
> > mfn no error should be returned to the caller. At the end the 'default
> > access' for that gfn -> mfn relation is the one established by
> > set_identity_p2m_entry and shouldn't be subject to the p2m default
> > access.
> 
> I think further reducing access is in theory supposed to be possible.

Couldn't an access reduction introduce issues, kind of similar to what
would happen if the regions are unmapped? (and that XSA-378 addressed)

I think whatever permissions set_identity_p2m_entry sets should be
mandatory ones, and no changes should be allowed. That would maybe
require introducing a new p2m type (p2m_mmio_mandatory) in order to
differentiate firmware mandatory MMIO mappings from the rest.

> For Dom0 all of this (including the potential of default access not
> being RWX) a questionable thing though, as pointed out in earlier
> discussions. After all there's no introspection (or alike) agent
> supposed to be controlling Dom0.

Ideally I would prefer a solution that could be applied to both dom0
and domU, if that's possible.

Thanks, Roger.