[Xen-devel] [PATCH v5 1/6] xen: xsm: flask: introduce XENMAPSPACE_gmfn_share for memory sharing

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

From: Zhongze Liu <blackskygg@xxxxxxxxx>

Author: Zhongze Liu <blackskygg@xxxxxxxxx>

The existing XENMAPSPACE_gmfn_foreign subop of XENMEM_add_to_physmap forbids
a Dom0 to map memory pages from one DomU to another, which restricts some useful
yet not dangerous use cases -- such as sharing pages among DomU's so that they
can do shm-based communication.

This patch introduces XENMAPSPACE_gmfn_share to address this inconvenience,
which is mostly the same as XENMAPSPACE_gmfn_foreign but has its own xsm check.

Specifically, the patch:

* Introduces a new av permission MMU__SHARE_MEM to denote if two domains can
  share memory by using the new subop;
* Introduces xsm_map_gmfn_share() to check if (current) has proper permission
  over (t) AND MMU__SHARE_MEM is allowed between (d) and (t);
* Modify the default xen.te to allow MMU__SHARE_MEM for normal domains that
  allow grant mapping/event channels.

The new subop is marked unsupported for x86 because calling p2m_add_foregin
on two DomU's is currently not supported on x86.

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>
Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>

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
---
Changes in v5:
- fix coding style
- remove useless x86 hypervisor message for the unimplemented op
- code style
- improve/add comments
---
 tools/flask/policy/modules/xen.if   |  2 ++
 xen/arch/arm/mm.c                   |  7 ++++++-
 xen/include/public/memory.h         |  8 ++++++++
 xen/include/xsm/dummy.h             | 15 +++++++++++++++
 xen/include/xsm/xsm.h               |  6 ++++++
 xen/xsm/dummy.c                     |  1 +
 xen/xsm/flask/hooks.c               | 12 ++++++++++++
 xen/xsm/flask/policy/access_vectors |  5 +++++
 8 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/tools/flask/policy/modules/xen.if 
b/tools/flask/policy/modules/xen.if
index 7aefd00..f841125 100644
--- a/tools/flask/policy/modules/xen.if
+++ b/tools/flask/policy/modules/xen.if
@@ -128,6 +128,8 @@ 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/arch/arm/mm.c b/xen/arch/arm/mm.c
index a6de77c..e5eb222 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1245,6 +1245,7 @@ int xenmem_add_to_physmap_one(
 
         break;
     case XENMAPSPACE_gmfn_foreign:
+    case XENMAPSPACE_gmfn_share:
     {
         struct domain *od;
         p2m_type_t p2mt;
@@ -1259,7 +1260,11 @@ int xenmem_add_to_physmap_one(
             return -EINVAL;
         }
 
-        rc = xsm_map_gmfn_foreign(XSM_TARGET, d, od);
+        if ( space == XENMAPSPACE_gmfn_foreign )
+            rc = xsm_map_gmfn_foreign(XSM_TARGET, d, od);
+        else
+            rc = xsm_map_gmfn_share(XSM_TARGET, d, od);
+
         if ( rc )
         {
             rcu_unlock_domain(od);
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index bf2f81f..a706e3c 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -227,6 +227,14 @@ DEFINE_XEN_GUEST_HANDLE(xen_machphys_mapping_t);
                                       Stage-2 using the Normal Memory
                                       Inner/Outer Write-Back Cacheable
                                       memory attribute. */
+#define XENMAPSPACE_gmfn_share   6 /* GMFN from another dom,
+                                      XENMEM_add_to_physmap_batch (and
+                                      currently ARM) only. Unlike
+                                      XENMAPSPACE_gmfn_foreign, it
+                                      requires current to have mapping
+                                      privileges instead of the
+                                      destination domain. */
+
 /* ` } */
 
 /*
diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
index ff6b2db..5064fce 100644
--- a/xen/include/xsm/dummy.h
+++ b/xen/include/xsm/dummy.h
@@ -535,6 +535,21 @@ static XSM_INLINE int xsm_map_gmfn_foreign(XSM_DEFAULT_ARG 
struct domain *d, str
     return xsm_default_action(action, d, t);
 }
 
+/*
+ * This action also requires that @current targets @d, but it has already been
+ * checked somewhere higher in the call stack.
+ *
+ * Be aware that this is not an exact default equivalence of its flask variant
+ * which also checks if @d and @t "are allowed to share memory pages", for we
+ * don't have a proper default equivalence of such a check.
+ */
+static XSM_INLINE int xsm_map_gmfn_share(XSM_DEFAULT_ARG struct domain *d,
+                                         struct domain *t)
+{
+    XSM_ASSERT_ACTION(XSM_TARGET);
+    return xsm_default_action(action, current->domain, t);
+}
+
 static XSM_INLINE int xsm_hvm_param(XSM_DEFAULT_ARG struct domain *d, unsigned 
long op)
 {
     XSM_ASSERT_ACTION(XSM_TARGET);
diff --git a/xen/include/xsm/xsm.h b/xen/include/xsm/xsm.h
index f0c6fc7..8873253 100644
--- a/xen/include/xsm/xsm.h
+++ b/xen/include/xsm/xsm.h
@@ -86,6 +86,7 @@ struct xsm_operations {
     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_share) (struct domain *d, struct domain *t);
     int (*claim_pages) (struct domain *d);
 
     int (*console_io) (struct domain *d, int cmd);
@@ -376,6 +377,11 @@ static inline int xsm_map_gmfn_foreign (xsm_default_t def, 
struct domain *d, str
     return xsm_ops->map_gmfn_foreign(d, t);
 }
 
+static inline int xsm_map_gmfn_share (xsm_default_t def, struct domain *d, 
struct domain *t)
+{
+    return xsm_ops->map_gmfn_share(d, t);
+}
+
 static inline int xsm_claim_pages(xsm_default_t def, struct domain *d)
 {
     return xsm_ops->claim_pages(d);
diff --git a/xen/xsm/dummy.c b/xen/xsm/dummy.c
index 6e75119..04e91d3 100644
--- a/xen/xsm/dummy.c
+++ b/xen/xsm/dummy.c
@@ -124,6 +124,7 @@ void __init xsm_fixup_ops (struct xsm_operations *ops)
     set_to_dummy_if_null(ops, add_to_physmap);
     set_to_dummy_if_null(ops, remove_from_physmap);
     set_to_dummy_if_null(ops, map_gmfn_foreign);
+    set_to_dummy_if_null(ops, map_gmfn_share);
 
     set_to_dummy_if_null(ops, vm_event_control);
 
diff --git a/xen/xsm/flask/hooks.c b/xen/xsm/flask/hooks.c
index 78bc326..250d476 100644
--- a/xen/xsm/flask/hooks.c
+++ b/xen/xsm/flask/hooks.c
@@ -1198,6 +1198,17 @@ 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);
 }
 
+/*
+ * This action also requires that @current has MMU__MAP_READ/WRITE over @d,
+ * but that has already been checked somewhere higher in the call stack (for
+ * example, by flask_add_to_physmap()).
+ */
+static int flask_map_gmfn_share(struct domain *d, struct domain *t)
+{
+    return current_has_perm(t, SECCLASS_MMU, MMU__MAP_READ | MMU__MAP_WRITE) ?:
+        domain_has_perm(d, t, SECCLASS_MMU, MMU__SHARE_MEM);
+}
+
 static int flask_hvm_param(struct domain *d, unsigned long op)
 {
     u32 perm;
@@ -1822,6 +1833,7 @@ static struct xsm_operations flask_ops = {
     .add_to_physmap = flask_add_to_physmap,
     .remove_from_physmap = flask_remove_from_physmap,
     .map_gmfn_foreign = flask_map_gmfn_foreign,
+    .map_gmfn_share = flask_map_gmfn_share,
 
 #if defined(CONFIG_HAS_PASSTHROUGH) && defined(CONFIG_HAS_PCI)
     .get_device_group = flask_get_device_group,
diff --git a/xen/xsm/flask/policy/access_vectors 
b/xen/xsm/flask/policy/access_vectors
index c5d8548..4a92252 100644
--- a/xen/xsm/flask/policy/access_vectors
+++ b/xen/xsm/flask/policy/access_vectors
@@ -385,6 +385,11 @@ class mmu
 # Allow a privileged domain to install a map of a page it does not own.  Used
 # for stub domain device models with the PV framebuffer.
     target_hack
+# Checked when using XENMEM_add_to_physmap with XENMAPSPACE_gmfn_share
+# to share memory between two domains:
+#  source = domain whose memory is being shared
+#  target = client domain
+    share_mem
 }
 
 # control of the paging_domctl split by subop
-- 
1.9.1


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