[Xen-devel] [PATCH] common/gnttab: Explicitly default to gnttab v1 during domain creation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

For reasons which appear to be exclusively down to poor review of the grant
table v2 code, a grant table's version field was wasn't initialised during
creation.

A number of problems (including XSAs) have occurred in the past trying trying
to use a grant table which hasn't been properly set up, and various areas of
the code cope with v0 by defaulting to v1.

In particular, the toolstack using GNTTABOP_setup_table to be able to fill in
the store/console grants has a side effect of switching to v1.

In hindsight however, this "fixup if we see 0" is a very poor, with a
substantial degree of risk.  Explicitly default to grant table v1 during
domain create, and let the rest of the code work safely in the knowledge that
the version is sensibly set.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Paul Durrant <paul.durrant@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

This probably wants backporting, so I chose not to integrate it into my series
which rearranges large chunks of DOMCTL_createdomain
---
 xen/common/grant_table.c | 40 +++++++---------------------------------
 1 file changed, 7 insertions(+), 33 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index d9ec711..8bae656 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -49,10 +49,7 @@ struct grant_table {
     percpu_rwlock_t       lock;
     /* Lock protecting the maptrack limit */
     spinlock_t            maptrack_lock;
-    /*
-     * The defined versions are 1 and 2.  Set to 0 if we don't know
-     * what version to use yet.
-     */
+    /* The defined versions are 1 and 2.  Any other value is invalid. */
     unsigned int          gt_version;
     /* Resource limits of the domain. */
     unsigned int          max_grant_frames;
@@ -222,7 +219,6 @@ nr_maptrack_frames(struct grant_table *t)
 static grant_entry_header_t *
 shared_entry_header(struct grant_table *t, grant_ref_t ref)
 {
-    ASSERT(t->gt_version != 0);
     if ( t->gt_version == 1 )
         return (grant_entry_header_t*)&shared_entry_v1(t, ref);
     else
@@ -1302,20 +1298,6 @@ unmap_common(
 
     grant_read_lock(rgt);
 
-    if ( rgt->gt_version == 0 )
-    {
-        /*
-         * This ought to be impossible, as such a mapping should not have
-         * been established (see the nr_grant_entries(rgt) bounds check in
-         * gnttab_map_grant_ref()). Doing this check only in
-         * gnttab_unmap_common_complete() - as it used to be done - would,
-         * however, be too late.
-         */
-        rc = GNTST_bad_gntref;
-        flags = 0;
-        goto unlock_out;
-    }
-
     op->rd = rd;
     op->ref = map->ref;
 
@@ -1932,9 +1914,6 @@ gnttab_setup_table(
         goto unlock;
     }
 
-    if ( gt->gt_version == 0 )
-        gt->gt_version = 1;
-
     if ( (op.nr_frames > nr_grant_frames(gt) ||
           ((gt->gt_version > 1) &&
            (grant_to_status_frames(op.nr_frames) > nr_status_frames(gt)))) &&
@@ -2991,15 +2970,11 @@ 
gnttab_set_version(XEN_GUEST_HANDLE_PARAM(gnttab_set_version_t) uop)
 
     switch ( gt->gt_version )
     {
-    case 0:
-        if ( op.version == 2 )
-        {
     case 1:
-            /* XXX: We could maybe shrink the active grant table here. */
-            res = gnttab_populate_status_frames(currd, gt, 
nr_grant_frames(gt));
-            if ( res < 0)
-                goto out_unlock;
-        }
+        /* XXX: We could maybe shrink the active grant table here. */
+        res = gnttab_populate_status_frames(currd, gt, nr_grant_frames(gt));
+        if ( res < 0)
+            goto out_unlock;
         break;
     case 2:
         for ( i = 0; i < GNTTAB_NR_RESERVED_ENTRIES; i++ )
@@ -3594,6 +3569,8 @@ grant_table_create(
     percpu_rwlock_resource_init(&t->lock, grant_rwlock);
     spin_lock_init(&t->maptrack_lock);
 
+    t->gt_version = 1;
+
     /* Okay, install the structure. */
     t->domain = d;
     d->grant_table = t;
@@ -3869,9 +3846,6 @@ int gnttab_map_frame(struct domain *d, unsigned long idx, 
gfn_t gfn,
 
     grant_write_lock(gt);
 
-    if ( gt->gt_version == 0 )
-        gt->gt_version = 1;
-
     if ( gt->gt_version == 2 &&
          (idx & XENMAPIDX_grant_table_status) )
     {
-- 
2.1.4


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