[Xen-devel] [PATCH L1TF MDS GT v2 2/2] common/grant_table: harden version dependent accesses

[Norbert Manthey <nmanthey@xxxxxxxxx>]

Guests can issue grant table operations and provide guest controlled
data to them. This data is used as index for memory loads after bound
checks have been done. Depending on the grant table version, the
size of elements in containers differ. As the base data structure is
a page, the number of elements per page also differs. Consequently,
bound checks are version dependent, so that speculative execution can
happen in several stages, the bound check as well as the version check.

This commit mitigates cases where out-of-bound accesses could happen
due to the version comparison. In cases, where no different memory
locations are accessed on the code path that follow an if statement,
no protection is required. No different memory locations are accessed
in the following functions after a version check:

 * gnttab_setup_table: only calculated numbersi are used, and then
        function gnttab_grow_table is called, which is version protected

 * gnttab_transfer: the case that depends on the version check just gets
        into copying a page or not

 * acquire_grant_for_copy: the not fixed comparison is on the abort path
        and does not access other structures, and on the else branch
        accesses only structures that have been validated before

 * gnttab_set_version: all accessible data is allocated for both versions
        Furthermore, the functions gnttab_populate_status_frames and
        gnttab_unpopulate_status_frames received a block_speculation
        macro. Hence, this code will only be executed once the correct
        version is visible in the architectural state.

 * gnttab_release_mappings: this function is called only during domain
       destruction and control is not returned to the guest

 * mem_sharing_gref_to_gfn: speculation will be stoped by the second if
       statement, as that places a barrier on any path to be executed.

 * gnttab_get_status_frame_mfn: no version dependent check, because all
       accesses, except the gt->status[idx], do not perform index-based
       accesses, or speculative out-of-bound accesses in the
       gnttab_grow_table function call.

 * gnttab_usage_print: cannot be triggered by the guest

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>
---

Notes:
  v2:  Add block_speculation to gnttab_populate_status_frames and
           gnttab_unpopulate_status_frames
       Add block_speculation in gnttab_get_status_frame_mfn, to make sure
           we actually work with a version 2 grant table

 xen/common/grant_table.c | 37 +++++++++++++++++++++++++------------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -828,7 +828,7 @@ static int _set_status(const grant_entry_header_t *shah,
                        domid_t ldomid)
 {
 
-    if ( rgt_version == 1 )
+    if ( evaluate_nospec(rgt_version == 1) )
         return _set_status_v1(shah, rd, act, readonly, mapflag, ldomid);
     else
         return _set_status_v2(shah, status, rd, act, readonly, mapflag, 
ldomid);
@@ -983,9 +983,12 @@ map_grant_ref(
 
     /* This call also ensures the above check cannot be passed speculatively */
     shah = shared_entry_header(rgt, ref);
-    status = rgt->gt_version == 1 ? &shah->flags : &status_entry(rgt, ref);
     act = active_entry_acquire(rgt, ref);
 
+    /* Make sure we do not access memory speculatively */
+    status = evaluate_nospec(rgt->gt_version == 1) ? &shah->flags
+                                                 : &status_entry(rgt, ref);
+
     /* If already pinned, check the active domid and avoid refcnt overflow. */
     if ( act->pin &&
          ((act->domid != ld->domain_id) ||
@@ -1006,7 +1009,7 @@ map_grant_ref(
 
         if ( !act->pin )
         {
-            unsigned long gfn = rgt->gt_version == 1 ?
+            unsigned long gfn = evaluate_nospec(rgt->gt_version == 1) ?
                                 shared_entry_v1(rgt, ref).frame :
                                 shared_entry_v2(rgt, ref).full_page.frame;
 
@@ -1458,7 +1461,7 @@ unmap_common_complete(struct gnttab_unmap_common *op)
     act = active_entry_acquire(rgt, op->ref);
     sha = shared_entry_header(rgt, op->ref);
 
-    if ( rgt->gt_version == 1 )
+    if ( evaluate_nospec(rgt->gt_version == 1) )
         status = &sha->flags;
     else
         status = &status_entry(rgt, op->ref);
@@ -1651,6 +1654,10 @@ gnttab_populate_status_frames(struct domain *d, struct 
grant_table *gt,
     unsigned req_status_frames;
 
     req_status_frames = grant_to_status_frames(req_nr_frames);
+
+    /* Make sure, prior version checks are architectural visible */
+    block_speculation();
+
     for ( i = nr_status_frames(gt); i < req_status_frames; i++ )
     {
         if ( (gt->status[i] = alloc_xenheap_page()) == NULL )
@@ -1679,6 +1686,9 @@ gnttab_unpopulate_status_frames(struct domain *d, struct 
grant_table *gt)
 {
     unsigned int i;
 
+    /* Make sure, prior version checks are architectural visible */
+    block_speculation();
+
     for ( i = 0; i < nr_status_frames(gt); i++ )
     {
         struct page_info *pg = virt_to_page(gt->status[i]);
@@ -1790,7 +1800,7 @@ gnttab_grow_table(struct domain *d, unsigned int 
req_nr_frames)
     }
 
     /* Status pages - version 2 */
-    if ( gt->gt_version > 1 )
+    if ( evaluate_nospec(gt->gt_version > 1) )
     {
         if ( gnttab_populate_status_frames(d, gt, req_nr_frames) )
             goto shared_alloc_failed;
@@ -2286,7 +2296,7 @@ gnttab_transfer(
         grant_read_lock(e->grant_table);
         act = active_entry_acquire(e->grant_table, ref);
 
-        if ( e->grant_table->gt_version == 1 )
+        if ( evaluate_nospec(e->grant_table->gt_version == 1) )
         {
             grant_entry_v1_t *sha = &shared_entry_v1(e->grant_table, ref);
 
@@ -2347,7 +2357,7 @@ release_grant_for_copy(
     sha = shared_entry_header(rgt, gref);
     mfn = act->mfn;
 
-    if ( rgt->gt_version == 1 )
+    if ( evaluate_nospec(rgt->gt_version == 1) )
     {
         status = &sha->flags;
         td = rd;
@@ -2446,7 +2456,7 @@ acquire_grant_for_copy(
     shah = shared_entry_header(rgt, gref);
     act = active_entry_acquire(rgt, gref);
 
-    if ( rgt->gt_version == 1 )
+    if ( evaluate_nospec(rgt->gt_version == 1) )
     {
         sha2 = NULL;
         status = &shah->flags;
@@ -3265,7 +3275,7 @@ swap_grant_ref(grant_ref_t ref_a, grant_ref_t ref_b)
     if ( act_b->pin )
         PIN_FAIL(out, GNTST_eagain, "ref b %#x busy\n", ref_b);
 
-    if ( gt->gt_version == 1 )
+    if ( evaluate_nospec(gt->gt_version == 1) )
     {
         grant_entry_v1_t shared;
 
@@ -3814,7 +3824,7 @@ int mem_sharing_gref_to_gfn(struct grant_table *gt, 
grant_ref_t ref,
         rc = -EINVAL;
     else if ( ref >= nr_grant_entries(gt) )
         rc = -ENOENT;
-    else if ( gt->gt_version == 1 )
+    else if ( evaluate_nospec(gt->gt_version == 1) )
     {
         const grant_entry_v1_t *sha1 = &shared_entry_v1(gt, ref);
 
@@ -3836,7 +3846,7 @@ int mem_sharing_gref_to_gfn(struct grant_table *gt, 
grant_ref_t ref,
         rc = -ENXIO;
     else if ( !rc && status )
     {
-        if ( gt->gt_version == 1 )
+        if ( evaluate_nospec(gt->gt_version == 1) )
             *status = flags;
         else
             *status = status_entry(gt, ref);
@@ -3856,6 +3866,9 @@ static int gnttab_get_status_frame_mfn(struct domain *d,
 
     ASSERT(gt->gt_version == 2);
 
+    /* Make sure we have version equal to 2 even under speculation */
+    block_speculation();
+
     if ( idx >= nr_status_frames(gt) )
     {
         unsigned long nr_status;
@@ -3922,7 +3935,7 @@ int gnttab_map_frame(struct domain *d, unsigned long idx, 
gfn_t gfn, mfn_t *mfn)
 
     grant_write_lock(gt);
 
-    if ( gt->gt_version == 2 && (idx & XENMAPIDX_grant_table_status) )
+    if ( evaluate_nospec(gt->gt_version == 2) && (idx & 
XENMAPIDX_grant_table_status) )
     {
         idx &= ~XENMAPIDX_grant_table_status;
         status = true;
-- 
2.7.4




Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Ralf Herbrich
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879




_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel