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 9A601377EAF; Mon, 6 Jul 2026 10:52:58 +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=1783335181; cv=none; b=nAw6m1ouSawM9UY38k9q/7kfyxH9Wnh3gnTyPSneCyq1xCkHiiTQDFQ1xnwC/nNplncmk42HFdSZz90+LyrnC9njlvEkYSZ1Wv2vbP3NfyORiBip4nMyHoGuU7vV8GF0KTrNxoM9lUFcKF3MgGFr6YH1/PiQGoRCSliSQtmu4d0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783335181; c=relaxed/simple; bh=5PDQKPlKrlK5mkq4F/AqOm5P1FJi9r8quvEcLVadPkI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YGdwxrvIVukQ6lX5lxIw8DLeD41PJEwVUa0VOaCfqc6gBTabVx7dQN2a9Ogp1uVTUz6Ihgo+juNKwwcLP1BmJ2tdd/xFVdda7Rdgw4umepycg0pEPSNv912NyqCI2F6/h9tfdW+edVwvXVBlVJ9cLh7BIWmfnOXMUNiZclJhtCU= 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=Y86LUQ/8; 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="Y86LUQ/8" 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 6E9512BC4; Mon, 6 Jul 2026 03:52:53 -0700 (PDT) Received: from [10.164.148.41] (MacBook-Pro.blr.arm.com [10.164.148.41]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2F4B13F85F; Mon, 6 Jul 2026 03:52:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1783335177; bh=5PDQKPlKrlK5mkq4F/AqOm5P1FJi9r8quvEcLVadPkI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Y86LUQ/88wqr9q+XQgYX5M2rHKMDqwHVd78+V56i28vxi0bJqCg45smjItGSht6vM LhTmj1Ma7QeT9qBKxDeVirPdzgpCOF1LMTwfCAoG4OkTwOZr6+PrXWwP5PvS1/LvOO FD/IYsLJPBEPg9IDaQQJJIuEA9IW1pESzn+k4g/8= Message-ID: <39a13445-dc6f-4a75-92b8-f4a122c63b89@arm.com> Date: Mon, 6 Jul 2026 16:22:25 +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 v3 1/6] arm64: make huge_ptep_get handled unaligned addresses To: "David Hildenbrand (Arm)" , Andrew Morton Cc: muchun.song@linux.dev, osalvador@suse.de, ljs@kernel.org, liam@infradead.org, riel@surriel.com, vbabka@kernel.org, harry@kernel.org, jannh@google.com, lance.yang@linux.dev, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, apopple@nvidia.com, rcampbell@nvidia.com, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, ak@linux.intel.com, nao.horiguchi@gmail.com, mel@csn.ul.ie, j-nomura@ce.jp.nec.com, pfalcato@suse.de, tglx@kernel.org, dave.hansen@intel.com, jpoimboe@kernel.org, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable@vger.kernel.org References: <20260703114202.365553-1-dev.jain@arm.com> <20260703114202.365553-2-dev.jain@arm.com> <20260705003559.8b124d2b94b685cc2e4e77ae@linux-foundation.org> <8fdbe0d6-87fd-441c-b6d2-baac380f6fb3@arm.com> <4b4e8007-3747-457a-85cc-d1003e1c8fe2@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: <4b4e8007-3747-457a-85cc-d1003e1c8fe2@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 06/07/26 2:15 pm, David Hildenbrand (Arm) wrote: > On 7/5/26 10:08, Dev Jain wrote: >> >> >> On 05/07/26 1:05 pm, Andrew Morton wrote: >>> On Fri, 3 Jul 2026 11:41:54 +0000 Dev Jain wrote: >>> >>>> huge_ptep_get() can be handed a virtual address pointing to the middle of >>>> a contpmd/contpte mapped hugetlb folio (examples of callers are >>>> pagemap_hugetlb_range, page_mapped_in_vma). >>>> >>>> The arm64 helper rewalks the pgtables in find_num_contig to answer whether >>>> the huge pte we have maps a contpmd or a contpte hugetlb folio, and >>>> returns CONT_PMDS or CONT_PTES, so that it can collect a/d bits over the >>>> contiguous ptes. We can falsely return CONT_PTES instead of CONT_PMDS >>>> if the addr is not aligned. >>>> >>>> Fix this by aligning the pmdp pointer down to a contpmd base before >>>> checking equality with the passed huge pte pointer, to correctly answer >>>> whether the huge pte is the base of a contpmd block. >>>> >>>> Fixes: 29cb80519689 ("arm64: hugetlb: Cleanup huge_pte size discovery mechanisms") >>>> Cc: stable@vger.kernel.org >>> >>> Please describe the userspace-visible effects of bugs when fixing them. >>> Particularly when cc:stable is proposed. Thanks. >> >> Forgot for this one. It should be, on systems where CONT_PTES != CONT_PMDS >> (meaning page size is 16K) we could collect excess a/d bit state, meaning >> extra work for the kernel. > > Even worse, right? We could walk 128 entries, when we really should just walk 16 > (IIRC) entries, possibly reading garbage or even worse, into a memory hole at > the end of memory? Hmm I was thinking that the checks pte_dirty() and pte_young() wouldn't care whether the pte is garbage. But, we could actually dereference a ptep pointer not having backing memory at all. Does the following sound good? "On systems where CONT_PTES != CONT_PMDS (meaning page size is 16K), we could collect excess a/d bit state, meaning extra work for the kernel. Even worse, we may iterate beyond the PTE table and dereference a garbage ptep pointer to access physical memory we don't own. Since the ptep pointer is a linear map address, we may run off the end of the linear map, dereference a VA not mapped into the kernel pgtables and cause kernel panic." Although I checked on arm64, there is no case in which there is a hole after the linear map, but still that assumption shouldn't be made. >