From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 911D126FA77; Sat, 7 Feb 2026 22:02:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770501756; cv=none; b=GVry0xeXkKxEUdgTRfvsZ56IA8TJ7msbIDpq9iF3Mw+IgtwGDcSui2XUyfi/ts8kTT8B9icw9ExyDBmiOjD+e36LOjt/PFhPCP4JBOWkQ7gn/O9sZ5Yp3LAdaB9+OIW99JftSWC0lEvgw02AQUrwPdQLeu5tbyDiW/ZyXN8YtEU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770501756; c=relaxed/simple; bh=lfcGFF8NdgjshgOUdzirLedNFsV9Zt9+VFJxLpuBxJM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nKapNiVU9qC0luYS2dhtNPCdLJHr0wqOWTQ7s6Ykwlg2sfEiAdpFMYM2pcMlDaolseXzorSGU2NDyF2vZH9FSNHnfi3UAuEXhKGY02WhNJsRcADu8KYwyk53Rc9BoghjjdyE6BLAjcbcESiAiJoPhxKFpDwBO621eczXjvVbpOw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YRqXTTbc; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YRqXTTbc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6812CC116D0; Sat, 7 Feb 2026 22:02:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770501756; bh=lfcGFF8NdgjshgOUdzirLedNFsV9Zt9+VFJxLpuBxJM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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