Re: [Xen-devel] [PATCH v7 10/10] xen/common: do not implicitly permit access to mapped I/O memory

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 26/05/14 12:37, Jan Beulich wrote:
> > > > On 26.05.14 at 13:24, <julien.grall@xxxxxxxxxx> wrote:
> > On 26/05/14 12:14, Jan Beulich wrote:
> > > > > > On 26.05.14 at 12:53, <julien.grall@xxxxxxxxxx> wrote:
> > > > On 26/05/14 11:14, Jan Beulich wrote:
> > > > > Or maybe I wasn't wrong - the patch context doesn't really make
> > > > > clear whether it's the granting or mapping operation that gets
> > > > > adjusted here (since an earlier patch moved the mapping one into
> > > > > this function).
> > > >             ret = -EPERM;
> > > > -        if ( !iomem_access_permitted(current->domain, mfn, mfn_end) )
> > > > +        if ( !iomem_access_permitted(d, mfn, mfn_end) )
> > > >                 break;
> > > >
> > > >             ret = xsm_iomem_mapping(XSM_HOOK, d, mfn, mfn_end, add);
> > > >
> > > > There is an xsm_iomem_mapping just after, so the change has been done in
> > > > XEN_DOMCTL_memory_mapping.
> > > In which case I indeed stick to my original comment - it's perhaps
> > > best to check _both_.
> > Why? We may want to map the region in the guest P2M without giving the
> > permission to the guest (I'm thinking about ARM passthrough case).
> How can you put a mapping of memory into a guest's P2M for which
> that guest has no access permission? To me this reads like you're
> intending to create a security issue here.
iomem_access_permitted is used to check if we allow the current guest to 
map a region in another guest P2M.
Once the mapping is done, at least on ARM, we don't use anymore the 
permission check. This is because there is no trap involved afterwards.
In a such case, I don't see any posssible security issue.

> > With your requirements, we have to call 2 hypercalls rather than one for
> > memory mapping, even if we don't want to allow the guest modifying iomem
> > range.
> While I can see you not allowing modification, even r/o access may
> (and likely will) be problematic for MMIO.
AFAIU, iomem_access_permitted is only here to allow modification of this 
range via hypercall.
Regards,

--
Julien Grall

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