gnttab: deal with status frame mapping race

Once gnttab_map_frame() drops the grant table lock, the MFN it reports
back to its caller is free to other manipulation. In particular
gnttab_unpopulate_status_frames() might free it, by a racing request on
another CPU, thus resulting in a reference to a deallocated page getting
added to a domain's P2M.

Obtain a page reference in gnttab_map_frame() to prevent freeing of the
page until xenmem_add_to_physmap_one() has actually completed its acting
on the page. Do so uniformly, even if only strictly required for v2
status pages, to avoid extra conditionals (which then would all need to
be kept in sync going forward).

This is CVE-2021-28701 / XSA-384.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
master commit: eb6bbf7b30da5bae87932514d54d0e3c68b23757
master date: 2021-09-08 14:37:45 +0200

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 38200e2533d5328f7359134c3aee649268c9dd20..e6c0a0d3534ffb6ea79c8206746da3798e45b35a 100644 (file)
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1423,6 +1423,8 @@ int xenmem_add_to_physmap_one(
         if ( rc )
             return rc;
 
+        /* Need to take care of the reference obtained in gnttab_map_frame(). */
+        page = mfn_to_page(mfn);
         t = p2m_ram_rw;
 
         break;
@@ -1490,9 +1492,12 @@ int xenmem_add_to_physmap_one(
     /* Map at new location. */
     rc = guest_physmap_add_entry(d, gfn, mfn, 0, t);
 
-    /* If we fail to add the mapping, we need to drop the reference we
-     * took earlier on foreign pages */
-    if ( rc && space == XENMAPSPACE_gmfn_foreign )
+    /*
+     * For XENMAPSPACE_gmfn_foreign if we failed to add the mapping, we need
+     * to drop the reference we took earlier. In all other cases we need to
+     * drop any reference we took earlier (perhaps indirectly).
+     */
+    if ( space == XENMAPSPACE_gmfn_foreign ? rc : page != NULL )
     {
         ASSERT(page != NULL);
         put_page(page);

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 56bad5b3ea07e2fff3caf5cd49924377a6369ef4..689a430d3b408de613631267c80c342156f5c9bc 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4620,6 +4620,8 @@ int xenmem_add_to_physmap_one(
             rc = gnttab_map_frame(d, idx, gpfn, &mfn);
             if ( rc )
                 return rc;
+            /* Need to take care of the ref obtained in gnttab_map_frame(). */
+            page = mfn_to_page(mfn);
             break;
         case XENMAPSPACE_gmfn:
         {

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 86e298dfc0499c9a15fbc6f563e76173e2cfda7f..f8486c3de78c41eaaac74f249f7b68baad1cdafd 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -4105,7 +4105,16 @@ int gnttab_map_frame(struct domain *d, unsigned long idx, gfn_t gfn, mfn_t *mfn)
     }
 
     if ( !rc )
-        gnttab_set_frame_gfn(gt, status, idx, gfn);
+    {
+        /*
+         * Make sure gnttab_unpopulate_status_frames() won't (successfully)
+         * free the page until our caller has completed its operation.
+         */
+        if ( get_page(mfn_to_page(*mfn), d) )
+            gnttab_set_frame_gfn(gt, status, idx, gfn);
+        else
+            rc = -EBUSY;
+    }
 
     grant_write_unlock(gt);