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 0E31118DB26; Wed, 15 Apr 2026 15:48:43 +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=1776268127; cv=none; b=Dc/+FGTiK+yLmJTctQbKhJcToPEDbm/eWahnYhhdLS5pICJPQIEOv7fLnWstAr/5Bp1L3cPmqUKxhZubGoRjdXpB05nHkHoIu9T+0LSG27oLXbFAXwxc8L++EdwdxB7Cx8CWXIxlHpeoBxNbAeFQ+c4iq45pbe0/tUc6lHGIiZg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776268127; c=relaxed/simple; bh=q7BNlaChEu4Coq4O1gsyhmPwaN89GJg8i1jtXzMvv/k=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=K88/QhKCodPq/f2TSu+yZWlCnKLcnfmZ02WFwkFSxDFGrNtMIQFOdV4r+CB3SbGc/np/au36uqxrkRMGuL2kvgO4i3GMg1yQWnR9vybLlI70G+CFXUlHGR9laiBINGrbWG/rUgEbTfa5E9C6vr5hTgVGJ4GrFOtdhex9YcqKoAU= 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=DMbIgYaF; 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="DMbIgYaF" 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 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 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 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