mm: sanitize page->mapping for tail pages

We don't define meaning of page->mapping for tail pages.  Currently it's
always NULL, which can be inconsistent with head page and potentially
lead to problems.

Let's poison the pointer to catch all illigal uses.

page_rmapping(), page_mapping() and page_anon_vma() are changed to look
on head page.

The only illegal use I've caught so far is __GPF_COMP pages from sound
subsystem, mapped with PTEs.  do_shared_fault() is changed to use
page_rmapping() instead of direct access to fault_page->mapping.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 14 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f952f055fdcf..370d44a5e25b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1805,7 +1805,7 @@ static void __split_huge_page_refcount(struct page *page,
                 */
                 page_tail->_mapcount = page->_mapcount;
 
-                BUG_ON(page_tail->mapping);
+                BUG_ON(page_tail->mapping != TAIL_MAPPING);
                 page_tail->mapping = page->mapping;
 
                 page_tail->index = page->index + i;
diff --git a/mm/memory.c b/mm/memory.c
index d4e4d37c1989..f9360dde6967 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3096,7 +3096,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
          * pinned by vma->vm_file's reference.  We rely on unlock_page()'s
          * release semantics to prevent the compiler from undoing this copying.
          */
-        mapping = fault_page->mapping;
+        mapping = page_rmapping(fault_page);
         unlock_page(fault_page);
         if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
                 /*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ce63d603820f..d02d6436add0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -466,6 +466,7 @@ void prep_compound_page(struct page *page, unsigned int order)
         for (i = 1; i < nr_pages; i++) {
                 struct page *p = page + i;
                 set_page_count(p, 0);
+                p->mapping = TAIL_MAPPING;
                 set_compound_head(p, page);
         }
 }
@@ -856,6 +857,10 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
                 ret = 0;
                 goto out;
         }
+        if (page->mapping != TAIL_MAPPING) {
+                bad_page(page, "corrupted mapping in tail page", 0);
+                goto out;
+        }
         if (unlikely(!PageTail(page))) {
                 bad_page(page, "PageTail not set", 0);
                 goto out;
@@ -866,6 +871,7 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
         }
         ret = 0;
 out:
+        page->mapping = NULL;
         clear_compound_head(page);
         return ret;
 }
diff --git a/mm/util.c b/mm/util.c
index 2d28f7930043..8acb936a52c8 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -386,7 +386,9 @@ struct anon_vma *page_anon_vma(struct page *page)
 
 struct address_space *page_mapping(struct page *page)
 {
-        unsigned long mapping;
+        struct address_space *mapping;
+
+        page = compound_head(page);
 
         /* This happens if someone calls flush_dcache_page on slab page */
         if (unlikely(PageSlab(page)))
@@ -399,10 +401,10 @@ struct address_space *page_mapping(struct page *page)
                 return swap_address_space(entry);
         }
 
-        mapping = (unsigned long)page->mapping;
-        if (mapping & PAGE_MAPPING_FLAGS)
+        mapping = page->mapping;
+        if ((unsigned long)mapping & PAGE_MAPPING_FLAGS)
                 return NULL;
-        return page->mapping;
+        return mapping;
 }
 
 int overcommit_ratio_handler(struct ctl_table *table, int write,