[PATCH] zsmalloc: don't underflow size calculation in zs_obj_write()

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Do not mix class->size and object size during offsets/sizes
calculation in zs_obj_write().  Size classes can merge into
clusters, based on objects-per-zspage and pages-per-zspage
characteristics, so some size classes can store objects
smaller than class->size.  This becomes problematic when
object size is much smaller than class->size - we can determine
that object spans two physical pages, because we use a larger
class->size for this, while the actual object is much smaller
and fits one physical page, so there is nothing to write to
the second page and memcpy() size calculation underflows.

We always know the exact size in bytes of the object
that we are about to write (store), so use it instead of
class->size.

Reported-by: Igor Belousov <igor.b@beldev.am>
Cc: <stable@vger.kernel.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/zsmalloc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 70406ac94bbd..999b513c7fdf 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1233,19 +1233,19 @@ void zs_obj_write(struct zs_pool *pool, unsigned long handle,
 	class = zspage_class(pool, zspage);
 	off = offset_in_page(class->size * obj_idx);
 
-	if (off + class->size <= PAGE_SIZE) {
+	if (!ZsHugePage(zspage))
+		off += ZS_HANDLE_SIZE;
+
+	if (off + mem_len <= PAGE_SIZE) {
 		/* this object is contained entirely within a page */
 		void *dst = kmap_local_zpdesc(zpdesc);
 
-		if (!ZsHugePage(zspage))
-			off += ZS_HANDLE_SIZE;
 		memcpy(dst + off, handle_mem, mem_len);
 		kunmap_local(dst);
 	} else {
 		/* this object spans two pages */
 		size_t sizes[2];
 
-		off += ZS_HANDLE_SIZE;
 		sizes[0] = PAGE_SIZE - off;
 		sizes[1] = mem_len - sizes[0];
 
-- 
2.49.0.906.g1f30a19c02-goog