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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 299E5C433EF for ; Tue, 19 Jun 2018 15:20:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BBA5B20652 for ; Tue, 19 Jun 2018 15:20:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BBA5B20652 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937931AbeFSPUC (ORCPT ); Tue, 19 Jun 2018 11:20:02 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:52190 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937471AbeFSPUA (ORCPT ); Tue, 19 Jun 2018 11:20:00 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5C49280D; Tue, 19 Jun 2018 08:20:00 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 2CE523F557; Tue, 19 Jun 2018 08:20:00 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id C8C871AE3797; Tue, 19 Jun 2018 16:20:35 +0100 (BST) Date: Tue, 19 Jun 2018 16:20:35 +0100 From: Will Deacon To: Ard Biesheuvel Cc: YaoJun , Kernel Hardening , Catalin Marinas , linux-arm-kernel , Linux Kernel Mailing List Subject: Re: [PATCH] arm64: mm: mark tramp_pg_dir read-only Message-ID: <20180619152035.GM13984@arm.com> References: <20180530044806.18449-1-yaojun8558363@gmail.com> <20180530091457.GB2452@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ard, Sorry, I forgot to reply to this. On Wed, May 30, 2018 at 11:53:20AM +0200, Ard Biesheuvel wrote: > On 30 May 2018 at 11:14, Will Deacon wrote: > > On Wed, May 30, 2018 at 12:48:06PM +0800, YaoJun wrote: > >> To protect against KSMA(Kernel Space Mirroring Attack), make > >> tramp_pg_dir read-only. The principle of KSMA is to insert a > >> carefully constructed PGD entry into the translation table. > >> The type of this entry is block, which maps the kernel text > >> and its access permissions bits are 01. The user process can > >> then modify kernel text directly through this mapping. In this > >> way, an arbitrary write can be converted to multiple arbitrary > >> writes. > >> > >> Signed-off-by: YaoJun > >> --- > >> arch/arm64/mm/mmu.c | 4 ++++ > >> 1 file changed, 4 insertions(+) > >> > >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > >> index 2dbb2c9f1ec1..ac4b22c7e435 100644 > >> --- a/arch/arm64/mm/mmu.c > >> +++ b/arch/arm64/mm/mmu.c > >> @@ -551,6 +551,10 @@ static int __init map_entry_trampoline(void) > >> __create_pgd_mapping(tramp_pg_dir, pa_start, TRAMP_VALIAS, PAGE_SIZE, > >> prot, pgd_pgtable_alloc, 0); > >> > >> + update_mapping_prot(__pa_symbol(tramp_pg_dir), > >> + (unsigned long)tramp_pg_dir, > >> + PGD_SIZE, PAGE_KERNEL_RO); > > > > Hmm, I like the idea but is there a risk that the page table has been mapped > > as part of a block entry, which we can't safely split at this point (i.e. > > we'll run into one of the BUG_ONs in the mapping code)? > > > > We'd need to create a separate segment for it initially so the mapping > is already at the right granularity. Why do you think that's the case? I can't see anything that guarantees this for the page table itself. Will