x86, mm: Hold mm->page_table_lock while doing vmalloc_sync

Take mm->page_table_lock while syncing the vmalloc region.  This prevents
a race with the Xen pagetable pin/unpin code, which expects that the
page_table_lock is already held.  If this race occurs, then Xen can see
an inconsistent page type (a page can either be read/write or a pagetable
page, and pin/unpin converts it between them), which will cause either
the pin or the set_p[gm]d to fail; either will crash the kernel.

vmalloc_sync_all() should be called rarely, so this extra use of
page_table_lock should not interfere with its normal users.

The mm pointer is stashed in the pgd page's index field, as that won't
be otherwise used for pgds.

Reported-by: Ian Campbell <ian.cambell@eu.citrix.com>
Originally-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4CB88A4C.1080305@goop.org>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 17 insertions, 3 deletions

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 5c4ee422590e..c70e57dbb491 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -87,7 +87,19 @@ static inline void pgd_list_del(pgd_t *pgd)
 #define UNSHARED_PTRS_PER_PGD                           \
         (SHARED_KERNEL_PMD ? KERNEL_PGD_BOUNDARY : PTRS_PER_PGD)
 
-static void pgd_ctor(pgd_t *pgd)
+
+static void pgd_set_mm(pgd_t *pgd, struct mm_struct *mm)
+{
+        BUILD_BUG_ON(sizeof(virt_to_page(pgd)->index) < sizeof(mm));
+        virt_to_page(pgd)->index = (pgoff_t)mm;
+}
+
+struct mm_struct *pgd_page_get_mm(struct page *page)
+{
+        return (struct mm_struct *)page->index;
+}
+
+static void pgd_ctor(struct mm_struct *mm, pgd_t *pgd)
 {
         /* If the pgd points to a shared pagetable level (either the
            ptes in non-PAE, or shared PMD in PAE), then just copy the
@@ -105,8 +117,10 @@ static void pgd_ctor(pgd_t *pgd)
         }
 
         /* list required to sync kernel mapping updates */
-        if (!SHARED_KERNEL_PMD)
+        if (!SHARED_KERNEL_PMD) {
+                pgd_set_mm(pgd, mm);
                 pgd_list_add(pgd);
+        }
 }
 
 static void pgd_dtor(pgd_t *pgd)
@@ -272,7 +286,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
          */
         spin_lock_irqsave(&pgd_lock, flags);
 
-        pgd_ctor(pgd);
+        pgd_ctor(mm, pgd);
         pgd_prepopulate_pmd(mm, pgd, pmds);
 
         spin_unlock_irqrestore(&pgd_lock, flags);