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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wgjwM-0000000CgWH-0yru; Mon, 06 Jul 2026 14:04:54 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wgjwL-0000000CgW8-2WxF for linux-arm-kernel@lists.infradead.org; Mon, 06 Jul 2026 14:04:53 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 1BCC740A20; Mon, 6 Jul 2026 14:04:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9AC5F1F000E9; Mon, 6 Jul 2026 14:04:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783346693; bh=3HB/QsivMV6qWr9y2CPSHM/Jiflu/9yz1TINj7TpNXM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LVRF9RdIrTzGO0Bfsx7LCsKDeyf7GKmtw9uAODtySDa9Zd1uxJMLkW1alzCidCpKV VV7j2iriGN1wWGo+cBvGIj4/n/87u54zHGRQHNSzMz/2CcYVOgoEJWl4Sywp5uK19G P+IdwMVkiVejxuPAnRxnc7VtXTlYW6zxRCqiUaY2TF6Bmzlq+QwX3+jzJoka9Zvy8I dOGTFq6y3jwJYOYnj9+CzPPw/kRAnFHlq5xIIhavv9jT/4K0tOKzSDsFUIwVjfF5jd WN8ieiYcggZUjYwWP4/WnvVuQxV+FxBj/BYtoCfxNXnCAfY4PzMoz0ejP4LDX8tynn np2O1anyHPS8A== Message-ID: <2b9d60a2-8698-46b0-80ae-b8747a66a85b@kernel.org> Date: Mon, 6 Jul 2026 16:04:40 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/6] arm64: make huge_ptep_get handled unaligned addresses To: Dev Jain , 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> <39a13445-dc6f-4a75-92b8-f4a122c63b89@arm.com> <7ac558fd-1081-456b-b997-c7321241cf8c@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 7/6/26 12:54, Dev Jain wrote: > > > On 06/07/26 4:22 pm, Dev Jain wrote: >> >> >> On 06/07/26 2:15 pm, David Hildenbrand (Arm) wrote: >>> >>> 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. > > Oh but we could access a linear map address which corresponds to a DRAM hole, meaning > there is no entry in the kernel pgtable. Yes, exactly. With debug-pagealloc and things like that we might have many holes in the direct map. -- Cheers, David