Re: [Xen-devel] [PATCH 4/6] xsm: flask: change the interface and default policy for xsm_map_gmfn_foregin

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 08/22/2017 10:18 PM, Zhongze Liu wrote:
> Hi Stefano,
>
> 2017-08-23 3:58 GMT+08:00 Stefano Stabellini <sstabellini@xxxxxxxxxx>:
> > On Wed, 23 Aug 2017, Zhongze Liu wrote:
> > > The original xsm_map_gmfn_foregin policy checks if source domain has the proper
> > > privileges over the target domain. Under this policy, it's not allowed if a Dom0
> > > wants to map pages from one DomU to another, this restricts some useful yet not
> > > dangerous usages of the API, such as sharing pages among DomU's by calling
> > > XENMEM_add_to_physmap from Dom0.
> > >
> > > Change the policy to: IIF current domain has the proper privilege on the
> >                          ^ IFF
> >
> >
> > > target domain and source domain, grant the access.
> > >
> > > References to this xsm check have also been updated to pass the current
> > > domain as a new parameter.
> > >
> > > This is for the proposal "Allow setting up shared memory areas between VMs
> > > from xl config file" (see [1]).
> > >
> > > [1] https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03047.html
> > >
> > > Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
> > >
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > > Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxx
> > > ---
> > >   xen/arch/arm/mm.c       | 2 +-
> > >   xen/arch/x86/mm/p2m.c   | 2 +-
> > >   xen/include/xsm/dummy.h | 6 ++++--
> > >   xen/include/xsm/xsm.h   | 7 ++++---
> > >   xen/xsm/flask/hooks.c   | 6 ++++--
> > >   5 files changed, 14 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > > index a810a056d7..9ec78d8c03 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -1284,7 +1284,7 @@ int xenmem_add_to_physmap_one(
> > >               return -EINVAL;
> > >           }
> > >
> > > -        rc = xsm_map_gmfn_foreign(XSM_TARGET, d, od);
> > > +        rc = xsm_map_gmfn_foreign(XSM_TARGET, current->domain, d, od);
> > >           if ( rc )
> > >           {
> > >               rcu_unlock_domain(od);
> > > diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> > > index e8a57d118c..a547fd00c0 100644
> > > --- a/xen/arch/x86/mm/p2m.c
> > > +++ b/xen/arch/x86/mm/p2m.c
> > > @@ -2545,7 +2545,7 @@ int p2m_add_foreign(struct domain *tdom, unsigned long 
> > > fgfn,
> > >       if ( tdom == fdom )
> > >           goto out;
> > >
> > > -    rc = xsm_map_gmfn_foreign(XSM_TARGET, tdom, fdom);
> > > +    rc = xsm_map_gmfn_foreign(XSM_TARGET, current->domain, tdom, fdom);
> > >       if ( rc )
> > >           goto out;
> > >
> > > diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
> > > index 62fcea6f04..28dbc6f2a2 100644
> > > --- a/xen/include/xsm/dummy.h
> > > +++ b/xen/include/xsm/dummy.h
> > > @@ -525,10 +525,12 @@ static XSM_INLINE int 
> > > xsm_remove_from_physmap(XSM_DEFAULT_ARG struct domain *d1,
> > >       return xsm_default_action(action, d1, d2);
> > >   }
> > >
> > > -static XSM_INLINE int xsm_map_gmfn_foreign(XSM_DEFAULT_ARG struct domain *d, 
> > > struct domain *t)
> > > +static XSM_INLINE int xsm_map_gmfn_foreign(XSM_DEFAULT_ARG struct domain *cd,
> > > +                                           struct domain *d, struct domain *t)
> > >   {
> > >       XSM_ASSERT_ACTION(XSM_TARGET);
> > > -    return xsm_default_action(action, d, t);
> > > +    return xsm_default_action(action, cd, d) ||
> > > +        xsm_default_action(action, cd, t);
> > We need to preserve the returned errors:
> >
> >    rc = xsm_default_action(action, cd, d);
> >    if (rc) return rc;
> >    return xsm_default_action(action, cd, t);
> OK, will correct this.
>
> > >   }
> > >
> > >   static XSM_INLINE int xsm_hvm_param(XSM_DEFAULT_ARG struct domain *d, 
> > > unsigned long op)
> > > diff --git a/xen/include/xsm/xsm.h b/xen/include/xsm/xsm.h
> > > index 60c0fd6a62..a20654a803 100644
> > > --- a/xen/include/xsm/xsm.h
> > > +++ b/xen/include/xsm/xsm.h
> > > @@ -85,7 +85,7 @@ struct xsm_operations {
> > >       int (*memory_pin_page) (struct domain *d1, struct domain *d2, struct 
> > > page_info *page);
> > >       int (*add_to_physmap) (struct domain *d1, struct domain *d2);
> > >       int (*remove_from_physmap) (struct domain *d1, struct domain *d2);
> > > -    int (*map_gmfn_foreign) (struct domain *d, struct domain *t);
> > > +    int (*map_gmfn_foreign) (struct domain *cd, struct domain *d, struct 
> > > domain *t);
> > >       int (*claim_pages) (struct domain *d);
> > >
> > >       int (*console_io) (struct domain *d, int cmd);
> > > @@ -372,9 +372,10 @@ static inline int xsm_remove_from_physmap(xsm_default_t 
> > > def, struct domain *d1,
> > >       return xsm_ops->remove_from_physmap(d1, d2);
> > >   }
> > >
> > > -static inline int xsm_map_gmfn_foreign (xsm_default_t def, struct domain *d, 
> > > struct domain *t)
> > > +static inline int xsm_map_gmfn_foreign (xsm_default_t def, struct domain *cd,
> > > +                                        struct domain *d, struct domain *t)
> > >   {
> > > -    return xsm_ops->map_gmfn_foreign(d, t);
> > > +    return xsm_ops->map_gmfn_foreign(cd, d, t);
> > >   }
> > >
> > >   static inline int xsm_claim_pages(xsm_default_t def, struct domain *d)
> > > diff --git a/xen/xsm/flask/hooks.c b/xen/xsm/flask/hooks.c
> > > index 91146275bb..3408b6b9e1 100644
> > > --- a/xen/xsm/flask/hooks.c
> > > +++ b/xen/xsm/flask/hooks.c
> > > @@ -1165,9 +1165,11 @@ static int flask_remove_from_physmap(struct domain *d1, 
> > > struct domain *d2)
> > >       return domain_has_perm(d1, d2, SECCLASS_MMU, MMU__PHYSMAP);
> > >   }
> > >
> > > -static int flask_map_gmfn_foreign(struct domain *d, struct domain *t)
> > > +static int flask_map_gmfn_foreign(struct domain *cd,
> > > +                                  struct domain *d, struct domain *t)
> > >   {
> > > -    return domain_has_perm(d, t, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE);
> > > +    return domain_has_perm(cd, d, SECCLASS_MMU, MMU__MAP_READ | 
> > > MMU__MAP_WRITE) ||
> > > +        domain_has_perm(cd, t, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE);
> > >   }
> > Same here:
> >
> >    rc = domain_has_perm(cd, d, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE);
> >    if (rc) return rc;
> >    return domain_has_perm(cd, t, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE);
> >
> > Also, I just want to point out that in the regular case cd and d are one
> > and the same. The code assumes that domain_has_perm returns 0 in that
> > case. I think that is correct, but I don't know enough about XSM to be
> > sure about it.
> I also assume that domain_has_perm returns 0 when cd == d, but let's
> wait for other
> maintainers' comments.
While this permission check with (cd == d) should succeed in all sane policies,
it's faster to compare for equality than to look up the access vector.

In addition, I think it would be useful to have a check that (d) and (t) can
share memory (so that a security policy could be written preventing them from
communicating directly).  Normally, this would be allowed between all domains
that allow grant mapping/event channels.

    rc = domain_has_perm(d, t, SECCLASS_MMU, MMU__SHARE_MEM);
    if (rc) return rc;

To allow this in the policy the same as grants:
--- a/tools/flask/policy/modules/xen.if
+++ b/tools/flask/policy/modules/xen.if
@@ -127,6 +127,8 @@ define(`domain_comms', `
        domain_event_comms($1, $2)
        allow $1 $2:grant { map_read map_write copy unmap };
        allow $2 $1:grant { map_read map_write copy unmap };
+       allow $1 $2:mmu share_mem;
+       allow $2 $1:mmu share_mem;
 ')


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