From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 59F392DEA93; Tue, 7 Jul 2026 19:25:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783452325; cv=none; b=Ot/bAMszUBLkx122K7zdFDZp95fDUtZbOvF6+SH5khcPfw0XLB+RStbNmLjwV7rwBUaVGdNYBCHJ95Bjz4A3S1zgThaNue0uRQy8ppMVLoWa51bthUTsqJTT40nnmslkdEtgMxd5l3dNT7KX9mJIQiu8G+zqZi+mzWpYiESHnNI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783452325; c=relaxed/simple; bh=qyHXkiiXtHVu6IXHR06nA/H0C6hdALcVPLtuIdOWBmc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=teKlwoTpjnXbpgwkduX3cWITEdDhiv8HDWzb6J5pQMX+r9lsORR4EknrQFZB4PEkGO6Bu9DEBM32Z/L+U7Wmv2VKWUQWhTgHbPFQ+An50Zv0S9URjF3M97lmCouXSHvq4rU6UMkrli0DnJXAL5xir50CK/kuR0LarHC2Yq/ctGI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Pnl3raGO; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Pnl3raGO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3BD0F1F000E9; Tue, 7 Jul 2026 19:25:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783452324; bh=2xhU7BWgC9NP48BtOkymuxeMVINmF7dxhGu9Uk05sPs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Pnl3raGOoJkCwbnzFm64lOMWPNyEnR+6u9+7fd989jtUVH/maOI9HvwzXCprAFwRd yj2xtf+KcrkVmrtUnSRrNgOANy/ipsDJGogspMH2MhUh12ZlWB4lWdCo9pX+nweAdq zs2xJSy2+eaiwb8qC8GMpJLuQIHKpAE4o4Q6Rqabc30+8O5qL6of8fUaQX6bH6XVbZ JY5W5kY9yUH9upf1RwbM2/HTnHc1lq7RTSqNmRCTAELq7TVY3KYYDn+3R50TZGi5YA SRrw+Bg7LkezQyWX75bJqhWDEw0EABo6/wu3+et1PoS/R3DZvJCuGArG/lCUN2pKk/ EwWpwozAAcx1w== Message-ID: Date: Tue, 7 Jul 2026 21:25:20 +0200 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] mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork() To: Kiryl Shutsemau Cc: akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@meta.com References: <20260703161833.57416-1-kirill@shutemov.name> <678ecae4-cda7-4683-9012-ed7c5c5b879f@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/7/26 19:05, Kiryl Shutsemau wrote: > On Tue, Jul 07, 2026 at 05:05:49PM +0200, David Hildenbrand (Arm) wrote: >> On 7/3/26 18:18, Kiryl Shutsemau wrote: >>> From: "Kiryl Shutsemau (Meta)" >>> >>> copy_hugetlb_page_range() clears the uffd-wp bit of hwpoison and >>> migration entries with huge_pte_clear_uffd_wp(), which operates on the >>> present-PTE bit position. Swap entries keep the uffd-wp state elsewhere >>> -- the same branches read and set it with pte_swp_uffd_wp() and >>> pte_swp_mkuffd_wp() -- and the present-PTE position falls into the swap >>> payload. On x86-64 it lands in the inverted swap offset, where a >>> naturally-aligned hugetlb PFN always has the affected bit set, so the >>> clear advances the encoded PFN by two pages. >>> >>> No userfaultfd needs to be involved: the clear is guarded only by the >>> child VMA not being uffd-wp registered, so a plain fork() with an >>> in-flight hugetlb migration entry (or a poisoned hugetlb page) corrupts >>> the entry copied into the child. Instrumenting the hwpoison branch and >>> forking after MADV_HWPOISON on a 2MB anon hugetlb page shows: >>> >>> offset before=120e00 >>> offset after =120e02 >>> >>> The fallout is mostly latent: rmap walks match migration entries by >>> folio range and remove_migration_pte() rebuilds the PTE from the folio, >>> so a within-folio PFN skew heals once migration completes. But any path >>> that re-encodes the corrupted offset -- e.g. hugetlb_change_protection() >>> rewriting a writable migration entry via >>> make_readable_migration_entry(swp_offset(entry)) -- propagates it, and >>> an hwpoison entry misidentifies which page is poisoned. >>> >>> Use pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and >>> move_huge_pte(). >>> >>> Reported-by: Sashiko AI review >>> Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@smtp.kernel.org/ >>> Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Kiryl Shutsemau >>> Assisted-by: Claude:claude-fable-5 >>> --- >>> mm/hugetlb.c | 4 ++-- >>> 1 file changed, 2 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 571212b80835..a4e6dd3a82f4 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -4918,7 +4918,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, >>> softleaf = softleaf_from_pte(entry); >>> if (unlikely(softleaf_is_hwpoison(softleaf))) { >>> if (!userfaultfd_wp(dst_vma)) >>> - entry = huge_pte_clear_uffd_wp(entry); >>> + entry = pte_swp_clear_uffd_wp(entry); >> >> I think installing a hwpoison pte will actually drop the uffd marker. >> >> hugetlb_change_protection() does nothing on hwpoison entrues. >> >> So how could be possibly get a hwpoison entry with an uffd-wp bit set here? >> >> If we indeed can't, Id assume there is nothing to clear here at all. > > You are right. > > I am inclined to remove it in a separate cleanup patch. Any objections? I'd just do it in this patch -- removing unnecessary code instead of fixing it. However, if there are strong opinions, I don't care that much. -- Cheers, David