[PATCH v2] mm: page_alloc: check the order of compound page even when the order is zero

[Hyesoo Yu]

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.

Reviewed-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
Signed-off-by: Hyesoo Yu <hyesoo.yu@xxxxxxxxxxx>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 95546f376302..fc92ac93c7c8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1078,6 +1078,7 @@ 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);
 
@@ -1096,16 +1097,15 @@ 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++) {
-- 
2.25.1