mm/z3fold: don't try to use buddy slots after free

As reported by Henry Burns:

Running z3fold stress testing with address sanitization showed zhdr->slots
was being used after it was freed.

  z3fold_free(z3fold_pool, handle)
    free_handle(handle)
      kmem_cache_free(pool->c_handle, zhdr->slots)
    release_z3fold_page_locked_list(kref)
      __release_z3fold_page(zhdr, true)
        zhdr_to_pool(zhdr)
          slots_to_pool(zhdr->slots)  *BOOM*

To fix this, add pointer to the pool back to z3fold_header and modify
zhdr_to_pool to return zhdr->pool.

Link: http://lkml.kernel.org/r/20190708134808.e89f3bfadd9f6ffd7eff9ba9@gmail.com
Fixes: 7c2b8baa  ("mm/z3fold.c: add structure for buddy handles")
Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Reported-by: Henry Burns <henryburns@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Jonathan Adams <jwadams@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -101,6 +101,7 @@ struct z3fold_buddy_slots {
  * @refcount:		reference count for the z3fold page
  * @work:		work_struct for page layout optimization
  * @slots:		pointer to the structure holding buddy slots
+ * @pool:		pointer to the containing pool
  * @cpu:		CPU which this page "belongs" to
  * @first_chunks:	the size of the first buddy in chunks, 0 if free
  * @middle_chunks:	the size of the middle buddy in chunks, 0 if free
@@ -114,6 +115,7 @@ struct z3fold_header {
 	struct kref refcount;
 	struct work_struct work;
 	struct z3fold_buddy_slots *slots;
+	struct z3fold_pool *pool;
 	short cpu;
 	unsigned short first_chunks;
 	unsigned short middle_chunks;
@@ -320,6 +322,7 @@ static struct z3fold_header *init_z3fold_page(struct page *page,
 	zhdr->start_middle = 0;
 	zhdr->cpu = -1;
 	zhdr->slots = slots;
+	zhdr->pool = pool;
 	INIT_LIST_HEAD(&zhdr->buddy);
 	INIT_WORK(&zhdr->work, compact_page_work);
 	return zhdr;
@@ -426,7 +429,7 @@ static enum buddy handle_to_buddy(unsigned long handle)
 
 static inline struct z3fold_pool *zhdr_to_pool(struct z3fold_header *zhdr)
 {
-	return slots_to_pool(zhdr->slots);
+	return zhdr->pool;
 }
 
 static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)