x86/mm: Rework get_page_and_type_from_mfn conditional

Make it easier to read by declaring the conditions in which we will
retain the ref, rather than the conditions under which we release it.

The only way (page == current->arch.old_guest_table) can be true is if
preemptible is true; so remove this from the query itself, and add an
ASSERT() to that effect on the opposite path.

No functional change intended.

NB that alloc_lN_table() mishandle the "linear pt failure" situation
described in the comment; this will be addressed in a future patch.

This is part of XSA-299.

Reported-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 68d17db4ad5e9bffa823f58955e731c77ac4af9e..da47a66f8b8b9a25baa4f9aaebb76462297753fe 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1092,8 +1092,43 @@ static int get_page_and_type_from_mfn(
 
     rc = _get_page_type(page, type, preemptible);
 
-    if ( unlikely(rc) && !partial_ref &&
-         (!preemptible || page != current->arch.old_guest_table) )
+    /*
+     * Retain the refcount if:
+     * - page is fully validated (rc == 0)
+     * - page is not validated (rc < 0) but:
+     *   - We came in with a reference (partial_ref)
+     *   - page is partially validated but there's been an error
+     *     (page == current->arch.old_guest_table)
+     *
+     * The partial_ref-on-error clause is worth an explanation.  There
+     * are two scenarios where partial_ref might be true coming in:
+     * - mfn has been partially demoted as type `type`; i.e. has
+     *   PGT_partial set
+     * - mfn has been partially demoted as L(type+1) (i.e., a linear
+     *   page; e.g. we're being called from get_page_from_l2e with
+     *   type == PGT_l1_table, but the mfn is PGT_l2_table)
+     *
+     * If there's an error, in the first case, _get_page_type will
+     * either return -ERESTART, in which case we want to retain the
+     * ref (as the caller will consider it retained), or -EINVAL, in
+     * which case old_guest_table will be set; in both cases, we need
+     * to retain the ref.
+     *
+     * In the second case, if there's an error, _get_page_type() can
+     * *only* return -EINVAL, and *never* set old_guest_table.  In
+     * that case we also want to retain the reference, to allow the
+     * page to continue to be torn down (i.e., PGT_partial cleared)
+     * safely.
+     *
+     * Also note that we shouldn't be able to leave with the reference
+     * count retained unless we succeeded, or the operation was
+     * preemptible.
+     */
+    if ( likely(!rc) || partial_ref )
+        /* nothing */;
+    else if ( page == current->arch.old_guest_table )
+        ASSERT(preemptible);
+    else
         put_page(page);
 
     return rc;