x86: don't store possibly stale TLB flush time stamp

While the timing window is extremely narrow, it is theoretically
possible for an update to the TLB flush clock and a subsequent flush
IPI to happen between the read and write parts of the update of the
per-page stamp. Exclude this possibility by disabling interrupts
across the update, preventing the IPI to be serviced in the middle.

This is XSA-241.

Reported-by: Jann Horn <jannh@google.com>
Suggested-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
master commit: 23a183607a
master date: 2017-10-12 14:48:25 +0200

xen/arch/arm/smp.c

@@ -1,4 +1,5 @@
 #include <xen/config.h>
+#include <xen/mm.h>
 #include <asm/system.h>
 #include <asm/smp.h>
 #include <asm/cpregs.h>

xen/arch/x86/mm.c

@@ -2334,7 +2334,7 @@ static int _put_final_page_type(struct page_info *page, unsigned long type,
          */
         if ( !(shadow_mode_enabled(page_get_owner(page)) &&
                (page->count_info & PGC_page_table)) )
-            page->tlbflush_timestamp = tlbflush_current_time();
+            page_set_tlbflush_timestamp(page);
         wmb();
         page->u.inuse.type_info--;
     }
@@ -2344,7 +2344,7 @@ static int _put_final_page_type(struct page_info *page, unsigned long type,
                 (PGT_count_mask|PGT_validated|PGT_partial)) == 1);
         if ( !(shadow_mode_enabled(page_get_owner(page)) &&
                (page->count_info & PGC_page_table)) )
-            page->tlbflush_timestamp = tlbflush_current_time();
+            page_set_tlbflush_timestamp(page);
         wmb();
         page->u.inuse.type_info |= PGT_validated;
     }
@@ -2398,7 +2398,7 @@ static int _put_page_type(struct page_info *page, bool_t preemptible,
             if ( ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info) )
             {
                 /*
-                 * page_set_tlbflush_timestamp() accesses the same union
+                 * 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
@@ -2419,7 +2419,7 @@ static int _put_page_type(struct page_info *page, bool_t preemptible,
              */
             if ( !(shadow_mode_enabled(page_get_owner(page)) &&
                    (page->count_info & PGC_page_table)) )
-                page->tlbflush_timestamp = tlbflush_current_time();
+                page_set_tlbflush_timestamp(page);
         }
 
         if ( likely((y = cmpxchg(&page->u.inuse.type_info, x, nx)) == x) )

xen/arch/x86/mm/shadow/common.c

@@ -1583,7 +1583,7 @@ void shadow_free(struct domain *d, mfn_t smfn)
          * TLBs when we reuse the page.  Because the destructors leave the
          * contents of the pages in place, we can delay TLB flushes until
          * just before the allocator hands the page out again. */
-        sp->tlbflush_timestamp = tlbflush_current_time();
+        page_set_tlbflush_timestamp(sp);
         perfc_decr(shadow_alloc_count);
         page_list_add_tail(sp, &d->arch.paging.shadow.freelist);
         sp = next;

xen/common/page_alloc.c

@@ -881,7 +881,7 @@ static void free_heap_pages(
         /* If a page has no owner it will need no safety TLB flush. */
         pg[i].u.free.need_tlbflush = (page_get_owner(&pg[i]) != NULL);
         if ( pg[i].u.free.need_tlbflush )
-            pg[i].tlbflush_timestamp = tlbflush_current_time();
+            page_set_tlbflush_timestamp(&pg[i]);
 
         /* This page is not a guest frame any more. */
         page_set_owner(&pg[i], NULL); /* set_gpfn_from_mfn snoops pg owner */

xen/include/asm-arm/flushtlb.h

@@ -14,6 +14,11 @@ do {                                                                    \
 
 #define tlbflush_current_time()                 (0)
 
+static inline void page_set_tlbflush_timestamp(struct page_info *page)
+{
+    page->tlbflush_timestamp = tlbflush_current_time();
+}
+
 #if defined(CONFIG_ARM_32)
 # include <asm/arm32/flushtlb.h>
 #elif defined(CONFIG_ARM_64)

xen/include/asm-x86/flushtlb.h

@@ -24,6 +24,20 @@ DECLARE_PER_CPU(u32, tlbflush_time);
 
 #define tlbflush_current_time() tlbflush_clock
 
+static inline void page_set_tlbflush_timestamp(struct page_info *page)
+{
+    /*
+     * Prevent storing a stale time stamp, which could happen if an update
+     * to tlbflush_clock plus a subsequent flush IPI happen between the
+     * reading of tlbflush_clock and the writing of the struct page_info
+     * field.
+     */
+    ASSERT(local_irq_is_enabled());
+    local_irq_disable();
+    page->tlbflush_timestamp = tlbflush_current_time();
+    local_irq_enable();
+}
+
 /*
  * @cpu_stamp is the timestamp at last TLB flush for the CPU we are testing.
  * @lastuse_stamp is a timestamp taken when the PFN we are testing was last