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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, T_DKIMWL_WL_HIGH 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 339B4C46464 for ; Thu, 9 Aug 2018 23:37:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C25A9223A2 for ; Thu, 9 Aug 2018 23:37:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="O2q0SimQ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C25A9223A2 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=chromium.org 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 S1727772AbeHJCEo (ORCPT ); Thu, 9 Aug 2018 22:04:44 -0400 Received: from mail-ua1-f65.google.com ([209.85.222.65]:39137 "EHLO mail-ua1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727218AbeHJCEo (ORCPT ); Thu, 9 Aug 2018 22:04:44 -0400 Received: by mail-ua1-f65.google.com with SMTP id g18-v6so185933uam.6 for ; Thu, 09 Aug 2018 16:37:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=X1hC3589ieDdOVwFecZnz7CZquloTCqkLCsVOYNAHUI=; b=O2q0SimQDrUfJgCNKwyZ0WjgY64j9Q7BYtGVQojHJhByWcKpe6Zedgcf6oHSl5qcB3 D6m1oeEkYg9YGOxSF8u/ffLUOMrOF28D2LedMU/fF3ScT6jLiIFhGH4s9inAXxagE5yW iF3i0KoabxPuEVmVzovHjtmYBEmPPXaxl+6ig= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=X1hC3589ieDdOVwFecZnz7CZquloTCqkLCsVOYNAHUI=; b=RJT9OOgSJSSWKQNUdUSx/+fjvAUaKm22SmR0oPOnv014c6U2JUOptvO+0KGxLd40u/ n3tDxF3HrthtLShWz7rGmAMVHBz9Q2pNmmB057MlPQreEzqdjX6e5qL7Vy2ooK+nC9he HeyaCZYWI5IBNKvCDV1UHewa/lmZquB8i6oDJY6aidCVPC1ILjofroVzC9juHtH3a95X 544+3QIsW/MABCDcF1bsmNoRlIPMg/miQqvzFTGDl1Bz+iE4Wy76UEkfkKTaQqk+d4yh cQRtk/+aQDm7AalmhcQukOVF0KDnR5cF9PUY4XQmEqGr+DggzFWypf8Bh5M7cYeqjiCQ C6XQ== X-Gm-Message-State: AOUpUlFSpW2LBE5fm9fGd+RcvpOqIT2Pn+zrgBI10sLU6Rgs3Cn82Szt nn+/oI85qRMrf2vBVp7uISeq1OLO0tksgP9P5B5LUQ== X-Google-Smtp-Source: AA+uWPyJLse65XJuYTeT5cBPztoJ2uuaVFFSY8XMJFv+M6U66bn/ZsG7A5WBEmuCSMNjlknYnaEIKN0nn+H4WoxNjeI= X-Received: by 2002:ab0:4987:: with SMTP id e7-v6mr2874472uad.198.1533857854026; Thu, 09 Aug 2018 16:37:34 -0700 (PDT) MIME-Version: 1.0 References: <20180809171722.144325-1-swboyd@chromium.org> <20180809171722.144325-8-swboyd@chromium.org> <153385579866.220756.16086660810932774163@swboyd.mtv.corp.google.com> In-Reply-To: <153385579866.220756.16086660810932774163@swboyd.mtv.corp.google.com> From: Julius Werner Date: Thu, 9 Aug 2018 16:37:20 -0700 Message-ID: Subject: Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive access To: swboyd@chromium.org Cc: Julius Werner , Greg Kroah-Hartman , LKML , Wei-Ning Huang , Brian Norris , samuel@sholland.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Furthermore, I see that my system RAM excludes this coreboot table so it > doesn't fall into the bucket that CONFIG_STRICT_DEVMEM would find. Yes, that is intentional. We don't want the kernel to try to use that memory for anything else (since we want those tables to survive), so we mark them as reserved in the e820 map. > > (I guess an alternative would be to rewrite 'cbmem' to use > > /sys/bus/coreboot/devices if available to get its coreboot table > > information. But we'd still need to maintain the old path for > > backwards compatibility anyway, so that would really just make it more > > complicated.) > > This sounds like a good idea. Userspace reaching into /dev/mem is not > good from a kernel hardening perspective. That's why those strict devmem > configs exist. Can cbmem be updated to query information from device > drivers instead, so that we can enable CONFIG_IO_STRICT_DEVMEM as well? Well... problem is that cbmem doesn't just access the coreboot tables, it accesses more stuff. There is actually a larger memory region called CBMEM (that's what the utility is named after) which contains all sorts of random memory allocations that coreboot wanted to survive for the lifetime of the system. The coreboot table is one section in there, and it sort of serves as a directory for some of the others (although there's also just a general CBMEM directory... there's some redundancy there). But cbmem can also print some of the other CBMEM sections which it finds by querying the coreboot table, such as the firmware log or the boot timestamps. So the question is how we can get to that content if /dev/mem isn't available anymore. One option would be to just write separate kernel drivers to completely replace the cbmem utility (we already have one for the log, for example), but I think Linux generally doesn't want to have too much logic and parsing and stuff in kernel drivers. Another option is to add a driver that just exposes a sysfs file through which you could read (we don't need to write) the CBMEM area... but then we'd essentially want that to take absolute addresses because that's what the coreboot table pointers contain, so we would've just built /dev/mem by another name (for a restricted range). The nicest thing, really, would be if there was a way for a kernel driver to mark specific regions as "allowed" by /dev/mem. I don't suppose we'd be willing to introduce a mechanism like that?