From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 68835F4384E for ; Wed, 15 Apr 2026 15:48:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 925106B0005; Wed, 15 Apr 2026 11:48:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8D66E6B0088; Wed, 15 Apr 2026 11:48:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 7C4C26B0089; Wed, 15 Apr 2026 11:48:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 69F136B0005 for ; Wed, 15 Apr 2026 11:48:46 -0400 (EDT) Received: from smtpin03.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 1D8481A0515 for ; Wed, 15 Apr 2026 15:48:46 +0000 (UTC) X-FDA: 84661223052.03.7BB248F Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf19.hostedemail.com (Postfix) with ESMTP id F0E241A000E for ; Wed, 15 Apr 2026 15:48:43 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=arm.com header.s=foss header.b=DMbIgYaF; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf19.hostedemail.com: domain of kevin.brodsky@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=kevin.brodsky@arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1776268124; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=l7eo5PnigErr5A+bIS4XNb3qrzb+gPfedlXxBhsCZHM=; b=Bma9pRd/GNyWSzeqhzCjskjKDXQoL6Zv3ScPJj6idsA7EWfs0zvoMf4mJgd7RjTCt+oZX8 W6/pTb0Tkmw9TW3COK1ZT5ADKMhISRS0hJDSofVrPo76rSFapOlfa4dH6qZxioxLFPYiht fa7C/CdFYYMh/3EQk40VhvVYtGnPQYY= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=arm.com header.s=foss header.b=DMbIgYaF; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf19.hostedemail.com: domain of kevin.brodsky@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=kevin.brodsky@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1776268124; a=rsa-sha256; cv=none; b=g3MjIccFeag2tw6W2c/pFpv9rxMbWzbREsZc/a7dZqTfUuB6yn820GxSqDa0vIz2IWjPJX 98Iq2opTF1CHY5wgE4ncXraPZb3tUx4OP04b/MWzpmORd5lsMmYPHFfG69v6eSX92UzmNP BzYySLRTWuROyRDF/SrI//1XyTkZVVs= 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 25CC03543; Wed, 15 Apr 2026 08:48:37 -0700 (PDT) Received: from [10.57.61.135] (unknown [10.57.61.135]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3C78A3F7B4; Wed, 15 Apr 2026 08:48:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1776268122; bh=q7BNlaChEu4Coq4O1gsyhmPwaN89GJg8i1jtXzMvv/k=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=DMbIgYaFwTOq3fm0mM1HfV3aAfyVqPNkHo/C7xTdVgjWROMvy8UUGutiPoWK/C704 LZRQQZ8yCINbuJve20sIyA//d6Qugq0C8eTCbtk0LniDtNwhtgrcj/bsxqVTevq7wl l+N3deWQkSGe/IRCBoxRG46gZ3h9RqlhJmkqjyco= Message-ID: <7d46b6f7-239b-40e0-a488-045b56b45c1e@arm.com> Date: Wed, 15 Apr 2026 17:48:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 00/30] pkeys-based page table hardening To: "David Hildenbrand (Arm)" , linux-hardening@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Andy Lutomirski , Catalin Marinas , Dave Hansen , Ira Weiny , Jann Horn , Jeff Xu , Joey Gouly , Kees Cook , Linus Walleij , Lorenzo Stoakes , Marc Zyngier , Mark Brown , Matthew Wilcox , Maxwell Bland , "Mike Rapoport (IBM)" , Peter Zijlstra , Pierre Langlois , Quentin Perret , Rick Edgecombe , Ryan Roberts , Thomas Gleixner , Vlastimil Babka , Will Deacon , Yang Shi , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, x86@kernel.org References: <20260227175518.3728055-1-kevin.brodsky@arm.com> <1c8e2cd6-4b50-4891-8a2d-6a45623e805f@kernel.org> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: <1c8e2cd6-4b50-4891-8a2d-6a45623e805f@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: F0E241A000E X-Stat-Signature: ed1xhn5b9gsj4m4pd8aewomnwfyos17b X-HE-Tag: 1776268123-925013 X-HE-Meta: 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 c1eUMTgY fFvCsvnrAarId+88Q81WURlQMmKPHRxOb6xm+XsGovsNdxISGhSlLHEfrNIJ1ReDwAPVDp5w0SzIVHHT/jK6jF9ZKGi+LjO97pCZYcFh2Bt+cwThVPvCowVW8yQMrQV+WlUw1eyRMgiNmgmxB3J+iHrQIdhYzaYpQO1PBeTDvhOTckDZFRZBKPscaJ8/1/CiwQ+ifFXQg8JzIbS2rI+znsT3samHPuhMIYa0Dfe6qA1zU+5k= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 15/04/2026 14:48, David Hildenbrand (Arm) wrote: > On 2/27/26 18:54, Kevin Brodsky wrote: >> NEW in v6: support for large block mappings through a dedicated page table >> allocator (patch 14-17) > Heh, I had to read till the very end to realize that this is an RFC, and > then saw your other mail. > > I can recommend using b4 for patch management, where you can configure a > sticky prefix through > > b4 prep --set-prefixes RFC > > And using "b4 send" to automate all the rest. I certainly should... sorry for the confusion! >> Threat model >> ============ >> >> The proposed scheme aims at mitigating data-only attacks (e.g. >> use-after-free/cross-cache attacks). In other words, it is assumed that >> control flow is not corrupted, and that the attacker does not achieve >> arbitrary code execution. Nothing prevents the pkey register from being >> set to its most permissive state - the assumption is that the register >> is only modified on legitimate code paths. >> >> A few related notes: >> >> - Functions that set the pkey register are all implemented inline. >> Besides performance considerations, this is meant to avoid creating >> a function that can be used as a straightforward gadget to set the >> pkey register to an arbitrary value. >> >> - kpkeys_set_level() only accepts a compile-time constant as argument, >> as a variable could be manipulated by an attacker. This could be >> relaxed but it seems unlikely that a variable kpkeys level would be >> needed in practice. >> > I see a lot of value for that also as a debugging mechanism. I hear that > other people had private patches that would attempt to only map leaf > pages in the direct map in pte_offset_map_lock() and friends. I assume > there are some tricky bits to that (concurrent access to page tables). Indeed, this should be a much better solution, not only because it means a lot fewer TLBIs, but also because it is truly per-thread (so concurrency is not a concern). > What's the general take regarding the thread model you describe vs. MTE? I'd say quite similar, although corrupting pointers (specifically the tag bits) remains possible in a data-only attack, while corrupting the POR_EL1 register would require some control flow hijack (only constant values are written to POR_EL1). > Regarding use-after-free, I'd assume KASAN would achieve something > similar. And with MTE "reasonably" fast. Or what is the biggest > difference you see, there? For use-after-free specifically, yes that sounds about right. > I'd assume that one difference would be, that not even match-all > pointers could accidentally modify page tables. Yep that's pretty much what I tried to say above - with pkeys you have to corrupt a system register to bypass the protection. > In the future, would you think that both mechanisms (pkey PT table > protection + KASAN) would be active at the same time, or wouldn't there > really be a lot of value in having both enabled? I think these are fairly orthogonal, KASAN gives you probabilistic spatial+temporal safety for most allocations, while kpkeys restricts access to key data to a small set of functions. I don't think one reduces the usefulness of the other. Of course KASAN makes it harder to use an arbitrary pointer to write to page tables, but kpkeys gives a clear guarantee (assuming CFI is preserved). > [...] > >> >> Open questions >> ============== >> >> A few aspects in this RFC that are debatable and/or worth discussing: >> >> - Can the pkeys block allocator be abstracted into something more >> generic? This seems desirable considering other use-cases for changing >> attributes of regions of the linear map, but the handling of page >> tables while splitting may be difficult to integrate in a generic >> allocator. >> >> - There is currently no restriction on how kpkeys levels map to pkeys >> permissions. A typical approach is to allocate one pkey per level and >> make it writable at that level only. As the number of levels >> increases, we may however run out of pkeys, especially on arm64 (just >> 8 pkeys with POE). Depending on the use-cases, it may be acceptable to >> use the same pkey for the data associated to multiple levels. >> >> >> Any comment or feedback is highly appreciated, be it on the high-level >> approach or implementation choices! > How crucial would the dedicated page table allocator be for a first up > streamed version? > > Assuming we introduce this as a debugging feature first, it would be > perfectly reasonable to just disallow large block mappings in the direct > map when enabled. > > That means, we could merge basic support first and think about how to > deal with page tables in a different way with most of the pkey details > out of the picture. I think that makes perfect sense, at least on arm64 where it's just a matter of configuring force_pte_mapping() appropriately. I'm not sure whether there is such an option on x86, though. - Kevin