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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 56C10C43331 for ; Mon, 30 Mar 2020 14:00:11 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id A79F12073B for ; Mon, 30 Mar 2020 14:00:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A79F12073B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18299-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 15858 invoked by uid 550); 30 Mar 2020 14:00:02 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 15826 invoked from network); 30 Mar 2020 14:00:01 -0000 Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature To: Ard Biesheuvel , Will Deacon Cc: Mark Rutland , Catalin Marinas , Linux ARM , kernel-hardening@lists.openwall.com References: <20200329141258.31172-1-ardb@kernel.org> <20200330135121.GD10633@willie-the-truck> From: Robin Murphy Message-ID: <3d23aa1f-d92c-5e39-733d-ebd135757b8e@arm.com> Date: Mon, 30 Mar 2020 14:59:47 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit On 2020-03-30 2:53 pm, Ard Biesheuvel wrote: > On Mon, 30 Mar 2020 at 15:51, Will Deacon wrote: >> >> On Sun, Mar 29, 2020 at 04:12:58PM +0200, Ard Biesheuvel wrote: >>> When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with >>> different permissions (r-x for .text, r-- for .rodata, rw- for .data, >>> etc) are rounded up to 2 MiB so they can be mapped more efficiently. >>> In particular, it permits the segments to be mapped using level 2 >>> block entries when using 4k pages, which is expected to result in less >>> TLB pressure. >>> >>> However, the mappings for the bulk of the kernel will use level 2 >>> entries anyway, and the misaligned fringes are organized such that they >>> can take advantage of the contiguous bit, and use far fewer level 3 >>> entries than would be needed otherwise. >>> >>> This makes the value of this feature dubious at best, and since it is not >>> enabled in defconfig or in the distro configs, it does not appear to be >>> in wide use either. So let's just remove it. >>> >>> Signed-off-by: Ard Biesheuvel >>> --- >>> arch/arm64/Kconfig.debug | 13 ------------- >>> arch/arm64/include/asm/memory.h | 12 +----------- >>> drivers/firmware/efi/libstub/arm64-stub.c | 8 +++----- >>> 3 files changed, 4 insertions(+), 29 deletions(-) >> >> Acked-by: Will Deacon >> >> But I would really like to go a step further and rip out the block mapping >> support altogether so that we can fix non-coherent DMA aliases: >> >> https://lore.kernel.org/lkml/20200224194446.690816-1-hch@lst.de >> > > I'm not sure I follow - is this about mapping parts of the static > kernel Image for non-coherent DMA? Yikes, I hope not! The concern there is about block entries in the linear map; I'd assume kernel text/data means not-linear-map, and is thus a different kettle of fish anyway. Robin.