mm: make page ref count overflow check tighter and more explicit

CVE-2019-11487

We have a VM_BUG_ON() to check that the page reference count doesn't
underflow (or get close to overflow) by checking the sign of the count.

That's all fine, but we actually want to allow people to use a "get page
ref unless it's already very high" helper function, and we want that one
to use the sign of the page ref (without triggering this VM_BUG_ON).

Change the VM_BUG_ON to only check for small underflows (or _very_ close
to overflowing), and ignore overflows which have strayed into negative
territory.
Acked-by: Matthew Wilcox <willy@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(backported from commit f958d7b5)
[ Connor Kuehl: instead of pulling in a large and invasive commit
  that provides page_ref wrappers, simply call the wrapped functions
  instead. I.e., atomic_read vs page_ref_count ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

include/linux/mm.h

@@ -488,6 +488,10 @@ static inline void get_huge_page_tail(struct page *page)
 
 extern bool __get_page_tail(struct page *page);
 
+/* 127: arbitrary random number, small enough to assemble well */
+#define page_ref_zero_or_close_to_overflow(page) \
+	((unsigned int) atomic_read(&page->_count) + 127u <= 127u)
+
 static inline void get_page(struct page *page)
 {
 	if (unlikely(PageTail(page)))
@@ -497,7 +501,7 @@ static inline void get_page(struct page *page)
 	 * Getting a normal page or the head of a compound page
 	 * requires to already have an elevated page->_count.
 	 */
-	VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
+	VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
 	atomic_inc(&page->_count);
 }