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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 61326C433E0 for ; Tue, 23 Jun 2020 16:42:55 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 12B5D2070E for ; Tue, 23 Jun 2020 16:42:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 12B5D2070E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id A65B96B0007; Tue, 23 Jun 2020 12:42:54 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A17516B000A; Tue, 23 Jun 2020 12:42:54 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 905236B000C; Tue, 23 Jun 2020 12:42:54 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0180.hostedemail.com [216.40.44.180]) by kanga.kvack.org (Postfix) with ESMTP id 78A156B0007 for ; Tue, 23 Jun 2020 12:42:54 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 193AC1849F6E5 for ; Tue, 23 Jun 2020 16:42:54 +0000 (UTC) X-FDA: 76961045868.03.cloud81_5c07e8226e3d Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin03.hostedemail.com (Postfix) with ESMTP id 0A11C106CD6 for ; Tue, 23 Jun 2020 16:42:48 +0000 (UTC) X-HE-Tag: cloud81_5c07e8226e3d X-Filterd-Recvd-Size: 6954 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf01.hostedemail.com (Postfix) with ESMTP for ; Tue, 23 Jun 2020 16:42:47 +0000 (UTC) Received: from gaia (unknown [2.26.170.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6788C2070E; Tue, 23 Jun 2020 16:42:44 +0000 (UTC) Date: Tue, 23 Jun 2020 17:42:41 +0100 From: Catalin Marinas To: Peter Collingbourne Cc: Kevin Brodsky , Linux ARM , Will Deacon , Marc Zyngier , Vincenzo Frascino , Szabolcs Nagy , Richard Earnshaw , Andrey Konovalov , linux-mm@kvack.org, linux-arch@vger.kernel.org, Branislav Rankov , Dave P Martin Subject: Re: [PATCH 20/22] arm64: mte: Allow user control of the excluded tags via prctl() Message-ID: <20200623164211.GA5180@gaia> References: <20191211184027.20130-1-catalin.marinas@arm.com> <20191211184027.20130-21-catalin.marinas@arm.com> <20200622171716.GC10226@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: 0A11C106CD6 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jun 22, 2020 at 12:00:48PM -0700, Peter Collingbourne wrote: > On Mon, Jun 22, 2020 at 10:17 AM Catalin Marinas > wrote: > > On Mon, Dec 16, 2019 at 09:30:36AM -0800, Peter Collingbourne wrote: > > > On Mon, Dec 16, 2019 at 6:20 AM Kevin Brodsky wrote: > > > > In this patch, the default exclusion mask remains 0 (i.e. all tags can be generated). > > > > After some more discussions, Branislav and I think that it would be better to start > > > > with the reverse, i.e. all tags but 0 excluded (mask = 0xfe or 0xff). > > > > > > > > This should simplify the MTE setup in the early C runtime quite a bit. Indeed, if all > > > > tags can be generated, doing any heap or stack tagging before the > > > > PR_SET_TAGGED_ADDR_CTRL prctl() is issued can cause problems, notably because tagged > > > > addresses could end up being passed to syscalls. Conversely, if IRG and ADDG never > > > > set the top byte by default, then tagging operations should be no-ops until the > > > > prctl() is issued. This would be particularly useful given that it may not be > > > > straightforward for the C runtime to issue the prctl() before doing anything else. > > > > > > > > Additionally, since the default tag checking mode is PR_MTE_TCF_NONE, it would make > > > > perfect sense not to generate tags by default. > > > > > > This would indeed allow the early C runtime startup code to pass > > > tagged addresses to syscalls, [...] > > > but I don't think it would entirely free > > > the code from the burden of worrying about stack tagging. Either way, > > > any stack frames that are active at the point when the prctl() is > > > issued would need to be compiled without stack tagging, because > > > otherwise those stack frames may use ADDG to rematerialize a stack > > > object address, which may produce a different address post-prctl. [...] > > > Setting the exclude mask to 0xffff would at least make it more likely > > > for this problem to be detected, though. > > > > I thought it would be detected if we didn't have a 0xffff default > > exclude mask. With only tag 0 generated, any such problem could be > > hidden. > > I don't think that's the case, as long as you aren't using 0 as a > catch-all tag. Imagine that you have some hypothetical startup code > that looks like this: > > void init() { > bool called_prctl = false; > prctl(PR_SET_TAGGED_ADDR_CTRL, ...); // effect is to change > GCR_EL1.Excl from 0xffff to 1 > called_prctl = true; > } > > This may be compiled as something like (well, a real compiler wouldn't > compile it like this but rather use sp-relative stores or eliminate > the dead stores entirely, but imagine that the stores to called_prctl > are obfuscated somehow, e.g. in another translation unit): > > sub x19, sp, #16 > irg x19, x19 // compute a tag base for the function > addg x0, x19, #0, #1 // add tag offset for "called_prctl" > stzg x0, [x0] > bl prctl > addg x0, x19, #0, #1 // rematerialize "called_prctl" address > mov w1, #1 > strb w1, [x0] > ret > > The first addg will materialize a tag of 0 due to the default Excl > value, so the stzg will set the memory tag to 0. However, the second > addg will materialize a tag of 1 because of the new Excl value, which > will result in a tag fault in the strb instruction. > > This problem is less likely to be detected if we transition Excl from > 0 to 1. It will only be detected in the case where the irg instruction > produces a tag of 0xf, which would be incremented to 0 by the first > addg but to 1 by the second one. Thanks for the explanation. For some reason I thought ADDG would only be used once (per variable or frame). I now agree that a default exclude mask of 0xffff would catch such issues early. > > > If we change the default in this way, maybe it would be worth > > > considering flipping the meaning of the tag mask and have it be a mask > > > of tags to allow. That would be consistent with the existing behaviour > > > where userspace sets bits in tagged_addr_ctrl in order to enable > > > tagging features. > > > > The first question is whether the C runtime requires a default > > GCR_EL1.Excl mask of 0xffff (or 0xfffe) so that IRG, ADDG, SUBG always > > generate tag 0. If the runtime is fine with a default exclude mask of 0, > > I'm tempted to go back to an exclude mask for prctl(). > > > > (to me it feels more natural to use an exclude mask as it matches the > > ARM ARM definition but maybe I stare too much at the hardware specs ;)) > > I think that would be fine with me. With the transition from 0 to 1 > the above problem would still be detected, but only 1/16 of the time. > But if the problem exists in the early startup code which will be > executed many times during a typical system boot, it makes it likely > that the problem will be detected eventually. I'm not a big fan of hitting a problem 1/16 times, it makes debugging harder. So I'll stick to a default exclude mask of 0xffff, in which case it makes sense to invert the polarity for prctl() and make it an include mask (as in v4 of the patchset). Thanks. -- Catalin