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 9A78BC5CFC1 for ; Tue, 19 Jun 2018 15:49:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5F7A720020 for ; Tue, 19 Jun 2018 15:49:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5F7A720020 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 S966938AbeFSPt3 (ORCPT ); Tue, 19 Jun 2018 11:49:29 -0400 Received: from foss.arm.com ([217.140.101.70]:52914 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966597AbeFSPt0 (ORCPT ); Tue, 19 Jun 2018 11:49:26 -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 0AE111596; Tue, 19 Jun 2018 08:49:26 -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 D08A93F557; Tue, 19 Jun 2018 08:49:25 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id 80E481AE3797; Tue, 19 Jun 2018 16:50:01 +0100 (BST) Date: Tue, 19 Jun 2018 16:50:01 +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: <20180619155001.GP13984@arm.com> References: <20180530044806.18449-1-yaojun8558363@gmail.com> <20180530091457.GB2452@arm.com> <20180619152035.GM13984@arm.com> <20180619152805.GN13984@arm.com> <20180619153754.GO13984@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 On Tue, Jun 19, 2018 at 05:40:26PM +0200, Ard Biesheuvel wrote: > On 19 June 2018 at 17:37, Will Deacon wrote: > > On Tue, Jun 19, 2018 at 05:29:03PM +0200, Ard Biesheuvel wrote: > >> On 19 June 2018 at 17:28, Will Deacon wrote: > >> > On Tue, Jun 19, 2018 at 05:23:41PM +0200, Ard Biesheuvel wrote: > >> >> On 19 June 2018 at 17:20, Will Deacon wrote: > >> >> > 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. > >> >> > > >> >> > >> >> We'd need to pass NO_BLOCK_MAPPINGS to map_kernel_segment(), > >> >> obviously, but that shouldn't hurt since that segment is relatively > >> >> tiny anyway. > >> > > >> > Ah right, with NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS, I agree that we're good. > >> > Ideally, we'd move {idmap,swapper,tramp}_pg_dir into .rodata... > >> > > >> > >> idmap and tramp yes, but swapper needs to be modifiable at runtime, no? > > > > Right, but couldn't we swizzle the permissions in e.g. set_pmd? We could > > even predicate that on a sanity check of the prot. > > > > Swizzle the permissions of the entire .rodata segment? That sounds > doable, but there is a whole class of data that belongs in this > category, and I think PaX/grsecurity had an API for that (but I don't > think anyone is upstreaming that yet). So let's not reinvent that > wheel for swapper_pg_dir only. I wasn't thinking of the whole .rodata segment -- just the page containing the entry being modified, but ok. I was hoping to avoid getting involved with the PaX/grsec stuff as it seems rather toxic from what I've seen. Will