+ mm-page_alloc-clear-page-private-in-free_pages_prepare.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/page_alloc: clear page->private in free_pages_prepare()
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page_alloc-clear-page-private-in-free_pages_prepare.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-clear-page-private-in-free_pages_prepare.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Subject: mm/page_alloc: clear page->private in free_pages_prepare()
Date: Sat, 7 Feb 2026 22:36:14 +0500

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>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@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: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/page_alloc.c~mm-page_alloc-clear-page-private-in-free_pages_prepare
+++ a/mm/page_alloc.c
@@ -1429,6 +1429,7 @@ __always_inline bool free_pages_prepare(
 
 	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);
_

Patches currently in -mm which might be from mikhail.v.gavrilov@gmail.com are

mm-page_alloc-clear-page-private-in-free_pages_prepare.patch