Re: [Xen-devel] [PATCH v3 2/7] xsm: flask: change the dummy xsm policy and flask hook for map_gmfn_foregin

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 10/18/2017 10:36 PM, Zhongze Liu wrote:
> The original dummy xsm_map_gmfn_foregin 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, which restricts some useful yet not
> dangerous use cases of the API, such as sharing pages among DomU's by calling
> XENMEM_add_to_physmap from Dom0.
>
> For the dummy xsm_map_gmfn_foregin, change to policy to: IFF the current domain
> has the proper privileges on (d) and (t), grant the access.
>
> For the flask side: 1) Introduce a new av permission MMU__SHARE_MEM to denote if
> two domains can share memory through map_gmfn_foregin. 2) Change to hook to
> grant the access IFF the current domain has proper MMU privileges on (d) and 
> (t),
> and MMU__SHARE_MEM is allowed between (d) and (t). 3) Modify the default xen.te
> to allow MMU__SHARE_MEM for normal domains that allow grant mapping/event
> channels.
>
> This is for the proposal "Allow setting up shared memory areas between VMs
> from xl config file" (see [1]).
>
> [1] https://lists.xen.org/archives/html/xen-devel/2017-08/msg03242.html
>
> Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
>
> Cc: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxx
> ---
>    V3:
>    * Change several if statements to the GCC '... = a ?: b' extention.
>    * lookup the current domain in the hooks instead of passing it as an arg
> ---
>   tools/flask/policy/modules/xen.if   | 2 ++
>   xen/include/xsm/dummy.h             | 3 ++-
>   xen/xsm/flask/hooks.c               | 4 +++-
>   xen/xsm/flask/policy/access_vectors | 4 ++++
>   4 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/tools/flask/policy/modules/xen.if 
> b/tools/flask/policy/modules/xen.if
> index 55437496f6..3ffd1c6239 100644
> --- 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;
>   ')
>   
>   # domain_self_comms(domain)
> diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
> index b2cd56cdc5..65e7060ad5 100644
> --- a/xen/include/xsm/dummy.h
> +++ b/xen/include/xsm/dummy.h
> @@ -516,7 +516,8 @@ static XSM_INLINE int 
> xsm_remove_from_physmap(XSM_DEFAULT_ARG struct domain *d1,
>   static XSM_INLINE int xsm_map_gmfn_foreign(XSM_DEFAULT_ARG struct domain *d, 
> struct domain *t)
>   {
>       XSM_ASSERT_ACTION(XSM_TARGET);
> -    return xsm_default_action(action, d, t);
> +    return xsm_default_action(action, current->domain, d) ?:
> +        xsm_default_action(action, current->domain, t);
>   }
Same comment as below, the check between (current->domain) and (d) should
be redundant with one higher up in the call stack.  The check between
(current->domain) and (t) should remain, although this *does* result in a
relaxing of the existing permission checks on the call as Jan noted.

>   static XSM_INLINE int xsm_hvm_param(XSM_DEFAULT_ARG struct domain *d, 
> unsigned long op)
> diff --git a/xen/xsm/flask/hooks.c b/xen/xsm/flask/hooks.c
> index f01b4cfaaa..16103bafc9 100644
> --- a/xen/xsm/flask/hooks.c
> +++ b/xen/xsm/flask/hooks.c
> @@ -1199,7 +1199,9 @@ static int flask_remove_from_physmap(struct domain *d1, 
> struct domain *d2)
>   
>   static int flask_map_gmfn_foreign(struct domain *d, struct domain *t)
>   {
> -    return domain_has_perm(d, t, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE);
> +    return domain_has_perm(current->domain, d, SECCLASS_MMU, MMU__MAP_READ | 
> MMU__MAP_WRITE) ?:
> +        domain_has_perm(current->domain, t, SECCLASS_MMU, MMU__MAP_READ | 
> MMU__MAP_WRITE) ?:
> +        domain_has_perm(d, t, SECCLASS_MMU, MMU__SHARE_MEM);
>   }
This is at least partially redundant with the higher-level permission checks
needed to get to the xenmem_add_* functions (xatp_permission_check call in
xen/common/memory.c, for example).  That check already verifies the permission
for (current->domain) to modify (d)'s page tables.

The other two checks here look correct.

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