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 X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1B3F6C433ED for ; Wed, 5 May 2021 11:08:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D89BC613D8 for ; Wed, 5 May 2021 11:08:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232580AbhEELJe (ORCPT ); Wed, 5 May 2021 07:09:34 -0400 Received: from mx2.suse.de ([195.135.220.15]:36586 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232642AbhEELJd (ORCPT ); Wed, 5 May 2021 07:09:33 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 4C0F0B168; Wed, 5 May 2021 11:08:36 +0000 (UTC) To: Rick Edgecombe , dave.hansen@intel.com, luto@kernel.org, peterz@infradead.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, linux-hardening@vger.kernel.org, kernel-hardening@lists.openwall.com Cc: ira.weiny@intel.com, rppt@kernel.org, dan.j.williams@intel.com, linux-kernel@vger.kernel.org References: <20210505003032.489164-1-rick.p.edgecombe@intel.com> From: Vlastimil Babka Subject: Re: [PATCH RFC 0/9] PKS write protected page tables Message-ID: Date: Wed, 5 May 2021 13:08:35 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <20210505003032.489164-1-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On 5/5/21 2:30 AM, Rick Edgecombe wrote: > This is a POC for write protecting page tables with PKS (Protection Keys for > Supervisor) [1]. The basic idea is to make the page tables read only, except > temporarily on a per-cpu basis when they need to be modified. I’m looking for > opinions on whether people like the general direction of this in terms of > value and implementation. > > Why would people want this? > =========================== > Page tables are the basis for many types of protections and as such, are a > juicy target for attackers. Mapping them read-only will make them harder to > use in attacks. > > This protects against an attacker that has acquired the ability to write to > the page tables. It's not foolproof because an attacker who can execute > arbitrary code can either disable PKS directly, or simply call the same > functions that the kernel uses for legitimate page table writes. Yeah, it's a good idea. I've once used a similar approach locally during debugging a problem that appeared to be stray writes hitting page tables, and without PKS I indeed made the whole pages read-only when not touched by the designated code. > Why use PKS for this? > ===================== > PKS is an upcoming CPU feature that allows supervisor virtual memory > permissions to be changed without flushing the TLB, like PKU does for user > memory. Protecting page tables would normally be really expensive because you > would have to do it with paging itself. PKS helps by providing a way to toggle > the writability of the page tables with just a per-cpu MSR. I can see in patch 8/9 that you are flipping the MSR around individual operations on page table entries. In my patch I hooked making the page table writable to obtaining the page table lock (IIRC I had only the PTE level fully handled though). Wonder if that would be better tradeoff even for your MSR approach? Vlastimil > Performance impacts > =================== > Setting direct map permissions on whatever random page gets allocated for a > page table would result in a lot of kernel range shootdowns and direct map > large page shattering. So the way the PKS page table memory is created is > similar to this module page clustering series[2], where a cache of pages is > replenished from 2MB pages such that the direct map permissions and associated > breakage is localized on the direct map. In the PKS page tables case, a PKS > key is pre-applied to the direct map for pages in the cache. > > There would be some costs of memory overhead in order to protect the direct > map page tables. There would also be some extra kernel range shootdowns to > replenish the cache on occasion, from setting the PKS key on the direct map of > the new pages. I don’t have any actual performance data yet. > > This is based on V6 [1] of the core PKS infrastructure patches. PKS > infrastructure follow-on’s are planned to enable keys to be set to the same > permissions globally. Since this usage needs a key to be set globally > read-only by default, a small temporary solution is hacked up in patch 8. Long > term, PKS protected page tables would use a better and more generic solution > to achieve this. > > [1] > https://lore.kernel.org/lkml/20210401225833.566238-1-ira.weiny@intel.com/ > [2] > https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@intel.com > / > > Thanks, > > Rick > > > Rick Edgecombe (9): > list: Support getting most recent element in list_lru > list: Support list head not in object for list_lru > x86/mm/cpa: Add grouped page allocations > mm: Explicitly zero page table lock ptr > x86, mm: Use cache of page tables > x86/mm/cpa: Add set_memory_pks() > x86/mm/cpa: Add perm callbacks to grouped pages > x86, mm: Protect page tables with PKS > x86, cpa: PKS protect direct map page tables > > arch/x86/boot/compressed/ident_map_64.c | 5 + > arch/x86/include/asm/pgalloc.h | 6 + > arch/x86/include/asm/pgtable.h | 26 +- > arch/x86/include/asm/pgtable_64.h | 33 ++- > arch/x86/include/asm/pkeys_common.h | 8 +- > arch/x86/include/asm/set_memory.h | 23 ++ > arch/x86/mm/init.c | 40 +++ > arch/x86/mm/pat/set_memory.c | 312 +++++++++++++++++++++++- > arch/x86/mm/pgtable.c | 144 ++++++++++- > include/asm-generic/pgalloc.h | 42 +++- > include/linux/list_lru.h | 26 ++ > include/linux/mm.h | 7 + > mm/Kconfig | 6 +- > mm/list_lru.c | 38 ++- > mm/memory.c | 1 + > mm/swap.c | 7 + > mm/swap_state.c | 6 + > 17 files changed, 705 insertions(+), 25 deletions(-) >