Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751930AbaBRSKG (ORCPT ); Tue, 18 Feb 2014 13:10:06 -0500 Received: from mail-oa0-f47.google.com ([209.85.219.47]:36267 "EHLO mail-oa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750921AbaBRSKD (ORCPT ); Tue, 18 Feb 2014 13:10:03 -0500 MIME-Version: 1.0 In-Reply-To: <20140217123415.GA2182@e103592.cambridge.arm.com> References: <1392339850-18686-1-git-send-email-keescook@chromium.org> <1392339850-18686-3-git-send-email-keescook@chromium.org> <20140214162257.GB2331@e103592.cambridge.arm.com> <20140217123415.GA2182@e103592.cambridge.arm.com> Date: Tue, 18 Feb 2014 10:10:03 -0800 X-Google-Sender-Auth: 4UXdof9_g1Gs28moDaCI7QnRPwQ Message-ID: Subject: Re: [PATCH 2/2] ARM: mm: keep rodata non-executable From: Kees Cook To: Dave Martin Cc: Catalin Marinas , Will Deacon , Larry Bassel , Stephen Rothwell , Russell King , Nicolas Pitre , Ben Dooks , =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= , Grant Likely , Jiang Liu , Christoffer Dall , Laura Abbott , Marc Zyngier , Rob Herring , Vitaly Andrianov , "linux-arm-kernel@lists.infradead.org" , Simon Baatz , Jonathan Austin , Greg Kroah-Hartman , LKML , Santosh Shilimkar , Andrew Morton Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 17, 2014 at 4:34 AM, Dave Martin wrote: > On Fri, Feb 14, 2014 at 11:11:07AM -0800, Kees Cook wrote: >> On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin wrote: >> > On Thu, Feb 13, 2014 at 05:04:10PM -0800, Kees Cook wrote: >> >> Introduce "CONFIG_DEBUG_RODATA" to mostly match the x86 config, though >> >> the behavior is different: it depends on STRICT_KERNMEM_PERMS, which >> >> sets rodata read-only (but executable), where as this option additionally >> >> splits rodata from the kernel text (resulting in potentially more memory >> >> lost to padding) and sets it non-executable as well. The end result is >> >> that on builds with CONFIG_DEBUG_RODATA=y (like x86) the rodata with be >> >> marked purely read-only. >> > >> > This triggers an Oops in kexec, because we have a block of code in .text >> > which is a template for generating baremetal code to relocate the new >> > kernel, and some literal words are written into it before copying. >> >> You're writing into the text area? I would imagine that >> CONFIG_ARM_KERNMEM_PERMS would break that. However, that's not the >> right place to be building code -- shouldn't the module area be used >> for that? >> >> > Possibly this should be in .rodata, not .text. >> >> Well, rodata should be neither writable nor executable. > > We're not writing into code exactly. > > This code is never executed in-place in vmlinux. It gets copied, and > only copies are ever executed. > > Some pointers and offsets get poked into the code to configure it. > > I think it would be better simply to put the code in .rodata, and > poke paramaters into the copy, not the original -- but that's a bit > more awkward to code up, since the values can't be poked simply by > writing global variables. Okay, interesting. I'll be curious to see what the patch for this looks like. >> > There may be a few other instances of this kind of thing. >> >> This config will certainly find them! :) But, that's why it's behind a config. > > I haven't tested exhaustively, but it think this is sufficient for a > Tested-by. The patch does seem to be doing what it is intended to > do, and doesn't seem to be triggering false positives all over the > place. Great, thanks for taking the time to check on it! Should I send this to the patch tracker, or wait for more feedback? >> > Are you aware of similar situations on other arches? >> >> I think there were some problems a long time ago on x86 for rodata too. > > It would be good to get this kexec case fixed -- I'll try to hack up > a separate patch. Thanks! -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/