From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 82F4B2BD09; Tue, 13 Oct 2026 10:13:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791886430; cv=none; b=YNbQA60LDi2LXjrusL9fo0uauSVGZE2cw3Drtx6dEeFdkJp9zE/w4gIMfy2VWnC/inBx2Tvh2mqjXDE/KvA6j3PeaZaP1tHEr5dUKqKp3CB/ve5q3nmTVl3U9imOLl7na3RDtMsITLOKkltzRcxCSnZvlB9eAbTBPZClkOAtwF0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791886430; c=relaxed/simple; bh=frG0so7TQeI6J8IN5+TVXwZehcv6LBDQHtosnjB44ng=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UwqHpLfAXMm3OVNFz37sIN8t4YsFJN3OZI4EHx9wP6UTItIanrMn9Gzhu0vQFgwBikheQ2MzI7C1MIjSSF3jWrK9AB+jPztW32Ckr3wze3wluA0VVpUGNvKxrp3YE1yMx9No6IKG3RTDaXqfCl7ssgrD1lDq3hrctmLVS0KPEss= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=vPt/TJBM; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="vPt/TJBM" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4A0C4169C; Tue, 13 Oct 2026 03:13:45 -0700 (PDT) Received: from [10.164.19.62] (unknown [10.164.19.62]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CE6393F7B4; Tue, 13 Oct 2026 03:13:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1791886428; bh=frG0so7TQeI6J8IN5+TVXwZehcv6LBDQHtosnjB44ng=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=vPt/TJBM5bSkHglczz78hY7BXD6SECatQcMU54W0ej74QhW76cpEiJFbPOEw8K7JU WqyxnjPkS4K6NPE26B912TQjoDzXTTsDExrMgBstckn+8MUOIndkJq14UA0k5lNrJl ao3zLpmO6rM7bJBP3j5Np6l8ehJV6OeOT6g0fdYo= Message-ID: <6a10b61f-5c4f-4d04-9742-97a358147c39@arm.com> Date: Tue, 13 Oct 2026 15:43:41 +0530 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] arm64/mm: restrict transparent contpte mappings to valid PTEs To: Zhao Gongyi , catalin.marinas@arm.com, will@kernel.org Cc: ryan.roberts@arm.com, baolin.wang@linux.alibaba.com, lance.yang@linux.dev, mark.rutland@arm.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261013093600.54995-1-zhaogongyi@bytedance.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20261013093600.54995-1-zhaogongyi@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 13/10/26 3:06 pm, Zhao Gongyi wrote: > The transparent contpte layer treats PTE_CONT as software state: when a > PTE has PTE_CONT set, the contpte helpers may align the operation to a > CONT_PTE_SIZE block and operate on the whole block. That invariant only > makes sense for valid mappings. Explicit users such as hugetlb manage > their contiguous encodings separately. > > contpte_set_ptes() currently sets PTE_CONT from alignment alone. Batched > set_ptes() users can therefore install present-invalid PTEs with > PTE_CONT set, for example mprotect(PROT_NONE), NUMA hinting through > modify_prot_commit_ptes(), fork() copying a PROT_NONE large folio, or a > PMD split installing PAGE_NONE PTEs. > > This is not a hardware programming problem: invalid descriptors are not > translated, so the hardware ignores descriptor bits such as PTE_CONT. It > is a software-state problem. Some arm64 fast paths intentionally test > pte_cont() directly instead of pte_valid_cont(), for example commit > 9970a9a27ffc ("arm64: mm: implement the architecture-specific > test_and_clear_young_ptes()"). Fixing the producer side preserves these > fast paths and covers all consumers. > > In the observed failure, mprotect(PROT_NONE) installed a full > CONT_PTE_SIZE range of present-invalid PTEs with PTE_CONT set. > MADV_GUARD_INSTALL then zapped one PTE. The nr == 1 clear_full_ptes() > path only unfolds pte_valid_cont() entries, so the neighbouring > present-invalid entries kept PTE_CONT while the target entry became a > guard marker. > > clear_refs_pte_range() then reached the marker through: > > ptep_test_and_clear_young() > test_and_clear_young_ptes() > contpte_test_and_clear_young_ptes() > contpte_align_addr_ptep() > __ptep_test_and_clear_young() > > The marker changed from old_pte=0x00000000000047c0 to > new_pte=0x00000000000043c0 by clearing PTE_AF. For swap PTEs, bit 10 is > part of the swap type field, bits 6-10: 0x47c0 is SWP_PTE_MARKER with > offset 4, while 0x43c0 has type 15 and is decoded as an ordinary swap > entry. Zap later warned in swap_put_entries_direct() with a bad swap > entry. > > This is triggerable by a user process against its own address space. On > systems running with panic_on_warn, the warning becomes a denial of > service. > > Fix the root cause by making contpte_set_ptes() use the transparent > contpte encoding only for valid descriptors. After this change, the > transparent contpte layer preserves the invariant that pte_cont(pte) > implies pte_valid(pte), so fast consumers can keep using PTE_CONT as > sufficient evidence of a transparent contpte block. > > Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings") > Cc: stable@vger.kernel.org > Signed-off-by: Zhao Gongyi > --- > Validation on an arm64 QEMU/KVM KASAN setup with CONFIG_ARM64_4K_PAGES=y > and CONFIG_ARM64_CONTPTE=y, using an out-of-tree guard-region reproducer: > > populate a CONT_PTE_SIZE anonymous THP range > mprotect(PROT_NONE) on the range > MADV_GUARD_INSTALL on one PTE in that range > write "1" to /proc/self/clear_refs > restore access and remove the guard marker Thanks, could you please give the code of the reproducer. That will save us some time. > > Unfixed kernel: failed in the first iteration, with 65 bad swap > messages and a WARNING in swap_put_entries_direct() > This patch: completed 100 iterations without warnings > > Additional producer-side instrumentation confirmed the same invalid > batch state outside the mprotect reproducer: > > Unfixed fork(): copy_page_range() wrote out_cont=1 > Unfixed PMD split: split_huge_pmd_locked() wrote out_cont=1 > This patch: both paths wrote out_cont=0 > > arch/arm64/include/asm/pgtable.h | 5 ++++- > arch/arm64/mm/contpte.c | 7 +++++++ > 2 files changed, 11 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 6000905a2..d6246042b 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -1650,7 +1650,10 @@ extern void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long ad > /* > * The contpte APIs are used to transparently manage the contiguous bit in ptes > * where it is possible and makes sense to do so. The PTE_CONT bit is considered > - * a private implementation detail of the public ptep API (see below). > + * a private implementation detail of the public ptep API (see below). The > + * transparent contpte layer maintains the invariant that pte_cont(pte) implies > + * pte_valid(pte); explicit contiguous-encoding users such as hugetlb manage > + * their entries separately. > */ > extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr, > pte_t *ptep, pte_t pte); > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index 0acab179f..851fa2e44 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -463,6 +463,13 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr, > if (!mm_is_user(mm)) > return __set_ptes(mm, addr, ptep, pte, nr); > > + /* > + * Keep the producer-side invariant documented in pgtable.h true even > + * if a direct caller passes an invalid descriptor with PTE_CONT set. > + */ > + if (!pte_valid(pte)) > + return __set_ptes(mm, addr, ptep, pte_mknoncont(pte), nr); > + > end = addr + (nr << PAGE_SHIFT); > pfn = pte_pfn(pte); > prot = pte_pgprot(pte);