[PATCH 6.18 709/752] mm/page_alloc: clear page->private in free_pages_prepare()

Sasha Levin
2026-02-28T17:58:47Z

From: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

[ Upstream commit ac1ea219590c09572ed5992dc233bbf7bb70fef9 ]

Several subsystems (slub, shmem, ttm, etc.) use page->private but don't
clear it before freeing pages. When these pages are later allocated as
high-order pages and split via split_page(), tail pages retain stale
page->private values.

This causes a use-after-free in the swap subsystem. The swap code uses
page->private to track swap count continuations, assuming freshly
allocated pages have page->private == 0. When stale values are present,
swap_count_continued() incorrectly assumes the continuation list is valid
and iterates over uninitialized page->lru containing LIST_POISON values,
causing a crash:

KASAN: maybe wild-memory-access in range [0xdead000000000100-0xdead000000000107]
RIP: 0010:__do_sys_swapoff+0x1151/0x1860

Fix this by clearing page->private in free_pages_prepare(), ensuring all
freed pages have clean state regardless of previous use.

Link: https://lkml.kernel.org/r/20260207173615.146159-1-mikhail.v.gavrilov@gmail.com
Fixes: 3b8000ae185c ("mm/vmalloc: huge vmalloc backing pages should be split rather than compound")
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Suggested-by: Zi Yan <ziy@nvidia.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/page_alloc.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f44524e1c92ac..d76f0f60f080d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1429,6 +1429,7 @@ __always_inline bool __free_pages_prepare(struct page *page,

page_cpupid_reset_last(page);
page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
+ page->private = 0;
reset_page_owner(page, order);
page_table_check_free(page, order);
pgalloc_tag_sub(page, 1 << order);
--
2.51.0