mm: page_alloc: check the order of compound page even when the order is zero

For compound pages, the head sets the PG_head flag and the tail sets the
compound_head to indicate the head page.  If a user allocates a compound
page and frees it with a different order, the compound page information
will not be properly initialized.  To detect this problem,
compound_order(page) and the order argument are compared, but this is not
checked when the order argument is zero.  That error should be checked
regardless of the order.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hyesoo Yu <[email protected]>
Reviewed-by: Vishal Moola (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Hyesoo Yu

mm/page_alloc.c

@@ -1079,6 +1079,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	int bad = 0;
 	bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
 	bool init = want_init_on_free();
+	bool compound = PageCompound(page);
 
 	VM_BUG_ON_PAGE(PageTail(page), page);
 
@@ -1097,16 +1098,15 @@ static __always_inline bool free_pages_prepare(struct page *page,
 		return false;
 	}
 
+	VM_BUG_ON_PAGE(compound && compound_order(page) != order, page);
+
 	/*
 	 * Check tail pages before head page information is cleared to
 	 * avoid checking PageCompound for order-0 pages.
 	 */
 	if (unlikely(order)) {
-		bool compound = PageCompound(page);
 		int i;
 
-		VM_BUG_ON_PAGE(compound && compound_order(page) != order, page);
-
 		if (compound)
 			page[1].flags &= ~PAGE_FLAGS_SECOND;
 		for (i = 1; i < (1 << order); i++) {