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b=YRqXTTbcGx40NrSqDqzfY/onQI7Oo//mZmwZDyNY/maP0p4HOQPI7MDjeHSgpXcgX v4bKJmWQvAk51DKDvBbnvx4blrRUsLdNewnoFGTBaCzTFVqefb+Qd6bnQOadW/LaJB HdS1hvjs7Hd3pvTGf54NcqlYVq82l6vB3dnc/ncKmGKGU1Q1kaef2CPUrnXr1RMPpu RcJlSKs0EIE/Wm74rWbYYQro8YLZELp6JbrqWqPyT9f91QOBAsdVRE2HPNtBRfCgJU e9DY0ErMD7D29hs8O3KJprmdTIj/4zYBLY0xu/eaFy+GEJwsdY2YhbSrTVRgRTdxMl sQ6c+OWqZyJgg== Message-ID: Date: Sat, 7 Feb 2026 23:02:27 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare() To: Mikhail Gavrilov , linux-mm@kvack.org Cc: akpm@linux-foundation.org, vbabka@suse.cz, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, npiggin@gmail.com, linux-kernel@vger.kernel.org, kasong@tencent.com, hughd@google.com, chrisl@kernel.org, ryncsn@gmail.com, stable@vger.kernel.org, willy@infradead.org References: <209207FE-D3A9-4BE2-8DA7-9BE38A19F387@nvidia.com> <20260207173615.146159-1-mikhail.v.gavrilov@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Stat-Signature: 3sr9bjiq79zjgyaoctmmyxszdwb7s7a4 X-Rspam-User: X-Rspamd-Queue-Id: 60BCE160004 X-HE-Tag: 1770501757-600954 X-HE-Meta: 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 00TlAj3s /+Nv2NvfyVUVcYaEEJP23I7UE00l+SZGoYnuB/DAHUK1qclIHWWPqaOA55/bJIQ9QztWzqYVAM38Mfa504KHsb33Nc51Ven9oD4ewY9MGJdA2sEjJt/gOAXdvWpvqYczh2T/kjGGSZffhHI6CwtgZ51za4ASzd0HLbfrQX4ECZyQ9oK6wuqnZMcQyYALCf5VJ+1ZI7Ox5D+vX0dKgECvwaGYb0vxAnV5hsRfu79VZ6coI4Xz2AM8xO+W/BlP0fHQtgmSDeU1VdV1IVP/irLf0a0x2wY9zd/7BpMaEGS5N/6DLU4XhgbiHgoTi59CioDwydXbOlwjX8rJf2kdRGFacoTpuYsG2lrmSoHbe1egYO8IdEl7aQyEshLjHonQPkpE38R02aHrzuBhOAQ9OH54hc/VbIxlj1oZyrCj+xOm+tfg8noPRj8rVYwthFsR6+b5N4vQR X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/7/26 18:36, Mikhail Gavrilov wrote: Thanks! > 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. I could have sworn we discussed something like that already in the past. I recall that freeing pages with page->private set was allowed. Although I once wondered whether we should actually change that. > > Fixes: 3b8000ae185c ("mm/vmalloc: huge vmalloc backing pages should be split rather than compound") > Cc: stable@vger.kernel.org > Suggested-by: Zi Yan > Acked-by: Zi Yan > Signed-off-by: Mikhail Gavrilov > --- Next time, please don't send patches as reply to another thread; that way it can easily get lost in a bigger thread. You want to get peoples attention :) > mm/page_alloc.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index cbf758e27aa2..24ac34199f95 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1430,6 +1430,7 @@ __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; Should we be using set_page_private()? It's a bit inconsistent :) I wonder, if it's really just the split_page() problem, why not handle it there, where we already iterate over all ("tail") pages? diff --git a/mm/page_alloc.c b/mm/page_alloc.c index cbf758e27aa2..cbbcfdf3ed26 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3122,8 +3122,10 @@ void split_page(struct page *page, unsigned int order) VM_BUG_ON_PAGE(PageCompound(page), page); VM_BUG_ON_PAGE(!page_count(page), page); - for (i = 1; i < (1 << order); i++) + for (i = 1; i < (1 << order); i++) { set_page_refcounted(page + i); + set_page_private(page, 0); + } split_page_owner(page, order, 0); pgalloc_tag_split(page_folio(page), order, 0); split_page_memcg(page, order); But then I thought about "what does actually happen during an folio split". We had a check in __split_folio_to_order() that got removed in 4265d67e405a, for some undocumented reason (and the patch got merged with 0 tags :( ). I assume because with zone-device there was a way to now got ->private properly set. But we removed the safety check for all other folios. - /* - * page->private should not be set in tail pages. Fix up and warn once - * if private is unexpectedly set. - */ - if (unlikely(new_folio->private)) { - VM_WARN_ON_ONCE_PAGE(true, new_head); - new_folio->private = NULL; - } I would have thought that we could have triggered that check easily before. Why didn't we? Who would have cleared the private field of tail pages? @Zi Yan, any idea why the folio splitting code wouldn't have revealed a similar problem? -- Cheers, David