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 DC8DA3DA7E4 for ; Tue, 1 Sep 2026 15:13:56 +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=1788275638; cv=none; b=gUV1i4iR/maB07IhSXYgPBgGx+oPV5XyibRoR758i43ZHgc080jjxmXaSHkeLP1PxJSqDNSSNB/DCbHfxaFBmWiNA+iqZolXS6Vdl0QxL8cjqiRLKPNRkvnviQDR/QgIhnNjtB5fKK5mFXJLaECV+GcuBAAiJPpBLxhSf4dEOvQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788275638; c=relaxed/simple; bh=nK6FlW3g45sKJd+aIPw8dI6/B0rZvMxgX9mQz8Xfn3s=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=crZZITB+S4D5G/thbPkjN05/bzp7DXe2tx0tRW/69sfrx7b+1TXhaFJzg6ODMXu7gvxJBD8KS1Q3j3B8kp3BUCMG2kOpgTvjZH0DbW3R282SCoZ/8b9fvQ5bsRHFf0KwC8rblg9lygrCojVyJr1Byz9qEYQRwLfvm8OGA03RC/4= 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=H+5iwdpX; 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="H+5iwdpX" 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 723A01756; Tue, 1 Sep 2026 08:13:52 -0700 (PDT) Received: from localhost (a079125.arm.com [10.164.21.43]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 799573F8C6; Tue, 1 Sep 2026 08:13:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1788275636; bh=nK6FlW3g45sKJd+aIPw8dI6/B0rZvMxgX9mQz8Xfn3s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H+5iwdpXKE4XGF6oml+V8KINgoiIOTA0gGYvkNv480Kxz2sYmXaEkDplZlH2zNGpr RLvw1oLRdESqbsuKP+J87oJljXJ3jByvuFKCoSuqzFqZLgESfiMXpQOjwDN10x5/WO ZoJuX0clb4c5zpgPHzG1VKsrkZVNf0z3+OPivamU= Date: Tue, 1 Sep 2026 20:43:52 +0530 From: Linu Cherian To: Kevin Brodsky Cc: linux-hardening@vger.kernel.org, Andrew Morton , Andy Lutomirski , Catalin Marinas , Dave Hansen , "David Hildenbrand (Arm)" , Jann Horn , Jeff Xu , Joey Gouly , Kees Cook , Linus Walleij , Marc Zyngier , Mark Brown , Matthew Wilcox , Maxwell Bland , "Mike Rapoport (IBM)" , Peter Zijlstra , Pierre Langlois , =?iso-8859-1?Q?Pierre-Cl=E9ment?= Tosi , Quentin Perret , Rick Edgecombe , Ryan Roberts , Vlastimil Babka , Will Deacon , Yang Shi , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, x86@kernel.org, Ira Weiny , Lorenzo Stoakes , Thomas Gleixner Subject: Re: [PATCH RFC v9 00/25] pkeys-based page table hardening Message-ID: References: <20260818-kpkeys-v9-0-743ad31b2c8f@arm.com> Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Kindly please ignore this, sent the reply again by mistake. On Tue, Sep 01, 2026 at 08:32:13PM +0530, Linu Cherian wrote: > Hi Kevin, > > On Tue, Aug 18, 2026 at 03:08:42PM +0100, Kevin Brodsky wrote: > > [Sending during the merge window in case reviewers have spare > > cycles; I'm not aiming to have this series merged in v7.3.] > > > > This is a proposal to leverage protection keys (pkeys) to harden > > critical kernel data, by making it mostly read-only. The series includes > > a simple framework called "kpkeys" to manipulate pkeys for in-kernel use, > > as well as a page table hardening feature based on that framework, > > "kpkeys_hardened_pgtables". Both are implemented on arm64 as a proof of > > concept, but they are designed to be compatible with any architecture > > that supports pkeys. > > > > The proposed approach is a typical use of pkeys: the data to protect is > > mapped with a given pkey P, and the pkey register is initially > > configured to grant read-only access to P. Where the protected data > > needs to be written to, the pkey register is temporarily switched to > > grant write access to P on the current CPU. > > > > The key fact this approach relies on is that the target data is > > only written to via a limited and well-defined API. This makes it > > possible to explicitly switch the pkey register where needed, without > > introducing excessively invasive changes, and only for a small amount of > > trusted code. > > > > Page tables are chosen as an initial target because of their especially > > critical nature - a single write may result in arbitrary pages becoming > > accessible to any context (including userspace). In order to keep the > > series digestible for reviewers, this version focuses on functionality > > rather than performance, making it most suitable as a debug feature. The > > key trade-off is the requirement to PTE-map the linear map - see section > > "Protected page table allocation" for details. > > > > This series has similarities with the "PKS write protected page tables" > > series posted by Rick Edgecombe a few years ago [1] but it is not > > specific to x86/PKS - the approach is meant to be generic. > > > > This proposal (as of RFC v5) was presented at Linux Security Summit > > Europe 2025 [2]. > > > > [Table of contents] > > > > * kpkeys > > - pkey register management > > > > * kpkeys_hardened_pgtables > > - Protected page table allocation > > - kpkeys context switching > > - Performance > > - Limitations > > > > * This series > > - Branches > > > > * Threat model > > > > * Further use-cases > > > > * Open questions > > > > kpkeys > > ====== > > > > The use of pkeys involves two separate mechanisms: assigning a pkey to > > pages, and defining the pkeys -> permissions mapping via the pkey > > register. This is implemented through the following interface: > > > > - Pages are assigned a pkey in the linear map using set_memory_pkey(). > > This is sufficient for this series, but it is also plausible for > > higher-level allocators to support marking allocations with a given > > pkey. > > > > - The pkey register is configured based on a *kpkeys context*. kpkeys > > contexts are represented as simple integers that correspond to a given > > configuration, for instance: > > > > KPKEYS_CTX_DEFAULT: > > RW access to KPKEYS_PKEY_DEFAULT > > RO access to any other KPKEYS_PKEY_* > > > > KPKEYS_CTX_: > > RW access to KPKEYS_PKEY_DEFAULT > > RW access to KPKEYS_PKEY_ > > RO access to any other KPKEYS_PKEY_* > > > > Only pkeys that are managed by the kpkeys framework are impacted; > > permissions for other pkeys are left unchanged (this allows for other > > schemes using pkeys to be used in parallel, and arch-specific use of > > certain pkeys). > > - Adding some basic details on what a scheme and context is quite helpful. > > - Giving some hints (may be an example) on how multiple schemes and multiple contexts > play together would be quite helpful. > > May be adding a documentation covering these aspects would be helpful as > well. > > My understanding is that pkeys are being partitioned across different > contexts. But then the introduction of the term "scheme" looks bit confusing to me. > > > > > The current kpkeys context is changed by calling > > kpkeys_enter_context(), which will set the pkey register > > accordingly and return the original state. A > > ..snip > > > Open questions > > ============== > > > > A few aspects in this RFC that are debatable and/or worth discussing: > > > > - There is currently no restriction on how kpkeys contexts map to pkeys > > permissions. A typical approach is to allocate one pkey per context and > > make it writable in that context only. As the number of contexts > > Probably to avoid the assumption, may be we can we have something like > below > > For a pkey P, we could define > PKEY_P_PERM_CTXT_OTHERS //permission for pkey p in other contexts > PKEY_P_PERM_CTXT_SELF //permission for pkey p in self context > > With the assumption of one pkey mapped for every context, > the permission for the default context would look something like, > > PKEY_DEF_PERM_CTXT_SELF << PKEY_DEF_PKEY_SHIFT | > PKEY_CT0_PERM_CTXT_OTHERS << PKEY_CT0_PKEY_SHIFT | > PKEY_CT1_PERM_CTXT_OTHERS << PKEY_CT1_PKEY_SHIFT | > ...(for all valid contexts) > > where, > Permission key, PKEY_DEF is associated with context DEFAULT, > Permission key, PKEY_CT0 is associated with context CT0, > Permission key, PKEY_CT1 is associated with context CT1 > > > > 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 contexts. > > > > Lets say two contexts A and B, use the same pkey P as their permission matches. > But then, when we enter context A, permission for pkey P gets > relaxed, then that would relax permission for pages associated with > context B as well which is unintended ? > > As the hardware supports 16 pkeys, should we consider removing the limit > of 8 pkeys so that we can have unique pkeys for each context ? > > -- > Linu Cherian >