[Xen-changelog] [xen stable-4.9] x86: don't allow page_unlock() to drop the last type reference

[patchbot@xxxxxxx]

commit a2af47d9ebbf3aa0fc80f697b37df878242f36f6
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Oct 12 15:07:28 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 12 15:07:28 2017 +0200

    x86: don't allow page_unlock() to drop the last type reference
    
    Only _put_page_type() does the necessary cleanup, and hence not all
    domain pages can be released during guest cleanup (leaving around
    zombie domains) if we get this wrong.
    
    This is XSA-242.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 6410733a8a0dff2fe581338ff631670cf91889db
    master date: 2017-10-12 14:49:46 +0200
---
 xen/arch/x86/mm.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 4b3c2a1..6e09e03 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1923,7 +1923,11 @@ void page_unlock(struct page_info *page)
 
     do {
         x = y;
+        ASSERT((x & PGT_count_mask) && (x & PGT_locked));
+
         nx = x - (1 | PGT_locked);
+        /* We must not drop the last reference here. */
+        ASSERT(nx & PGT_count_mask);
     } while ( (y = cmpxchg(&page->u.inuse.type_info, x, nx)) != x );
 }
 
@@ -2611,6 +2615,17 @@ static int _put_page_type(struct page_info *page, bool 
preemptible,
                    (page->count_info & PGC_page_table)) )
                 page_set_tlbflush_timestamp(page);
         }
+        else if ( unlikely((nx & (PGT_locked | PGT_count_mask)) ==
+                           (PGT_locked | 1)) )
+        {
+            /*
+             * We must not drop the second to last reference when the page is
+             * locked, as page_unlock() doesn't do any cleanup of the type.
+             */
+            cpu_relax();
+            y = page->u.inuse.type_info;
+            continue;
+        }
 
         if ( likely((y = cmpxchg(&page->u.inuse.type_info, x, nx)) == x) )
             break;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.9

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