x86/grant: disallow misaligned PTEs

Pagetable entries must be aligned to function correctly.  Disallow attempts
from the guest to have a grant PTE created at a misaligned address, which
would result in corruption of the L1 table with largely-guest-controlled
values.

This is CVE-2017-12137 / XSA-227.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f53ca435543c069dde1e1ec6a7d414f3aaab957f..5b863c6fa64c0b4ed0ba4678238f6d8a21ff4ce3 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3767,6 +3767,9 @@ static int create_grant_pte_mapping(
     l1_pgentry_t ol1e;
     struct domain *d = v->domain;
 
+    if ( !IS_ALIGNED(pte_addr, sizeof(nl1e)) )
+        return GNTST_general_error;
+
     adjust_guest_l1e(nl1e, d);
 
     gmfn = pte_addr >> PAGE_SHIFT;
@@ -3823,6 +3826,16 @@ static int destroy_grant_pte_mapping(
     struct page_info *page;
     l1_pgentry_t ol1e;
 
+    /*
+     * addr comes from Xen's active_entry tracking so isn't guest controlled,
+     * but it had still better be PTE-aligned.
+     */
+    if ( !IS_ALIGNED(addr, sizeof(ol1e)) )
+    {
+        ASSERT_UNREACHABLE();
+        return GNTST_general_error;
+    }
+
     gmfn = addr >> PAGE_SHIFT;
     page = get_page_from_gfn(d, gmfn, NULL, P2M_ALLOC);