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On 6 Feb 2026, at 12:40, Mikhail Gavrilov wrote: > When vmalloc allocates high-order pages and splits them via split_page(), > tail pages may retain stale page->private values from previous use by the > buddy allocator. Do you have a reproducer for this issue? Last time I checked page->private usage, I find users clears ->private before free a page. I wonder which one I was missing. The comment above page_private() does say ->private can be used on tail pages. If pages are freed with non-zero private in tail pages, we need to either correct the violating user or clear all pages ->private in post_alloc_hook() in addition to the head one. Clearing ->private in split_page() looks like a hack instead of a fix. > > This causes a use-after-free in the swap subsystem. The swap code uses > vmalloc_to_page() to get struct page pointers for swap_map, then uses > page->private to track swap count continuations. In add_swap_count_ > continuation(), the condition "if (!page_private(head))" assumes fresh > pages have page->private =3D=3D 0, but tail pages from split_page() may h= ave > non-zero stale values. > > When page->private accidentally contains a value like SWP_CONTINUED (32), > swap_count_continued() incorrectly assumes the continuation list is valid > and iterates over uninitialized page->lru, which may contain LIST_POISON > values from a previous list_del(), causing a crash: > > KASAN: maybe wild-memory-access in range [0xdead000000000100-0xdead0000= 00000107] > RIP: 0010:__do_sys_swapoff+0x1151/0x1860 > > Fix this by clearing page->private for tail pages in split_page(). Note > that we don't touch page->lru to avoid breaking split_free_page() which > may have the head page on a list. > > Fixes: 3b8000ae185c ("mm/vmalloc: huge vmalloc backing pages should be sp= lit rather than compound") > Cc: stable@vger.kernel.org > Signed-off-by: Mikhail Gavrilov > --- > mm/page_alloc.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index cbf758e27aa2..3604a00e2118 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -3122,8 +3122,14 @@ void split_page(struct page *page, unsigned int or= der) > VM_BUG_ON_PAGE(PageCompound(page), page); > VM_BUG_ON_PAGE(!page_count(page), page); > > - for (i =3D 1; i < (1 << order); i++) > + for (i =3D 1; i < (1 << order); i++) { > set_page_refcounted(page + i); > + /* > + * Tail pages may have stale page->private from buddy > + * allocator or previous use. Clear it. > + */ > + set_page_private(page + i, 0); > + } > split_page_owner(page, order, 0); > pgalloc_tag_split(page_folio(page), order, 0); > split_page_memcg(page, order); > --=20 > 2.53.0 Best Regards, Yan, Zi