[Xen-changelog] [xen stable-4.6] x86: don't wrongly trigger linear page table assertion

[patchbot@xxxxxxx]

commit 8d3fe28cf48291adcec9d9b77661da16d09ba118
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Nov 16 12:14:13 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 16 12:14:13 2017 +0100

    x86: don't wrongly trigger linear page table assertion
    
    _put_page_type() may do multiple iterations until its cmpxchg()
    succeeds. It invokes set_tlbflush_timestamp() on the first
    iteration, however. Code inside the function takes care of this, but
    - the assertion in _put_final_page_type() would trigger on the second
      iteration if time stamps in a debug build are permitted to be
      sufficiently much wider than the default 6 bits (see WRAP_MASK in
      flushtlb.c),
    - it returning -EINTR (for a continuation to be scheduled) would leave
      the page inconsistent state (until the re-invocation completes).
    Make the set_tlbflush_timestamp() invocation conditional, bypassing it
    (for now) only in the case we really can't tolerate the stamp to be
    stored.
    
    This is part of XSA-240.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 23 +++++++----------------
 1 file changed, 7 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f897eb6..189ea17 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2498,29 +2498,20 @@ static int _put_page_type(struct page_info *page, 
bool_t preemptible,
                 break;
             }
 
-            if ( ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info) )
-            {
-                /*
-                 * set_tlbflush_timestamp() accesses the same union
-                 * linear_pt_count lives in. Unvalidated page table pages,
-                 * however, should occur during domain destruction only
-                 * anyway.  Updating of linear_pt_count luckily is not
-                 * necessary anymore for a dying domain.
-                 */
-                ASSERT(page_get_owner(page)->is_dying);
-                ASSERT(page->linear_pt_count < 0);
-                ASSERT(ptpg->linear_pt_count > 0);
-                ptpg = NULL;
-            }
-
             /*
              * Record TLB information for flush later. We do not stamp page
              * tables when running in shadow mode:
              *  1. Pointless, since it's the shadow pt's which must be tracked.
              *  2. Shadow mode reuses this field for shadowed page tables to
              *     store flags info -- we don't want to conflict with that.
+             * Also page_set_tlbflush_timestamp() accesses the same union
+             * linear_pt_count lives in. Pages (including page table ones),
+             * however, don't need their flush time stamp set except when
+             * the last reference is being dropped. For page table pages
+             * this happens in _put_final_page_type().
              */
-            if ( !(shadow_mode_enabled(page_get_owner(page)) &&
+            if ( (!ptpg || !PGT_type_equal(x, ptpg->u.inuse.type_info)) &&
+                 !(shadow_mode_enabled(page_get_owner(page)) &&
                    (page->count_info & PGC_page_table)) )
                 page_set_tlbflush_timestamp(page);
         }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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