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=-2.3 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 0F67BC433E0 for ; Mon, 1 Jun 2020 15:04:22 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C663F2065C for ; Mon, 1 Jun 2020 15:04:21 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C663F2065C 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 5FAD280007; Mon, 1 Jun 2020 11:04:21 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5ABEF8E0006; Mon, 1 Jun 2020 11:04:21 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4749980007; Mon, 1 Jun 2020 11:04:21 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0189.hostedemail.com [216.40.44.189]) by kanga.kvack.org (Postfix) with ESMTP id 2DAEC8E0006 for ; Mon, 1 Jun 2020 11:04:21 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id DF0161233C0 for ; Mon, 1 Jun 2020 15:04:20 +0000 (UTC) X-FDA: 76880963880.04.back28_7b5649a4f5361 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin04.hostedemail.com (Postfix) with ESMTP id F170A8032EAE for ; Mon, 1 Jun 2020 15:04:07 +0000 (UTC) X-HE-Tag: back28_7b5649a4f5361 X-Filterd-Recvd-Size: 6303 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf46.hostedemail.com (Postfix) with ESMTP for ; Mon, 1 Jun 2020 15:04:07 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A33AE1FB; Mon, 1 Jun 2020 08:04:06 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E858B3F305; Mon, 1 Jun 2020 08:04:04 -0700 (PDT) Date: Mon, 1 Jun 2020 16:04:02 +0100 From: Dave Martin To: Catalin Marinas Cc: linux-arch@vger.kernel.org, Vincenzo Frascino , Will Deacon , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , linux-mm@kvack.org, Linux ARM , nd@arm.com, Peter Collingbourne , Evgenii Stepanov Subject: Re: [PATCH v4 11/26] arm64: mte: Add PROT_MTE support to mmap() and mprotect() Message-ID: <20200601150402.GC5031@arm.com> References: <20200515171612.1020-1-catalin.marinas@arm.com> <20200515171612.1020-12-catalin.marinas@arm.com> <20200528091445.GA2961@gaia> <20200528110509.GA18623@arm.com> <20200528163412.GC2961@gaia> <20200601085536.GV5031@arm.com> <20200601144544.GC23419@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200601144544.GC23419@gaia> User-Agent: Mutt/1.5.23 (2014-03-12) X-Rspamd-Queue-Id: F170A8032EAE X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 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 01, 2020 at 03:45:45PM +0100, Catalin Marinas wrote: > On Mon, Jun 01, 2020 at 09:55:38AM +0100, Dave P Martin wrote: > > On Thu, May 28, 2020 at 05:34:13PM +0100, Catalin Marinas wrote: > > > On Thu, May 28, 2020 at 12:05:09PM +0100, Szabolcs Nagy wrote: > > > > The 05/28/2020 10:14, Catalin Marinas wrote: > > > > > On Wed, May 27, 2020 at 11:57:39AM -0700, Peter Collingbourne wrote: > > > > > > Should the userspace stack always be mapped as if with PROT_MTE if the > > > > > > hardware supports it? Such a change would be invisible to non-MTE > > > > > > aware userspace since it would already need to opt in to tag checking > > > > > > via prctl. This would let userspace avoid a complex stack > > > > > > initialization sequence when running with stack tagging enabled on the > > > > > > main thread. > > > > > > > > > > I don't think the stack initialisation is that difficult. On program > > > > > startup (can be the dynamic loader). Something like (untested): > > > > > > > > > > register unsigned long stack asm ("sp"); > > > > > unsigned long page_sz = sysconf(_SC_PAGESIZE); > > > > > > > > > > mprotect((void *)(stack & ~(page_sz - 1)), page_sz, > > > > > PROT_READ | PROT_WRITE | PROT_MTE | PROT_GROWSDOWN); > > > > > > > > > > (the essential part it PROT_GROWSDOWN so that you don't have to specify > > > > > a stack lower limit) > > > > > > > > does this work even if the currently mapped stack is more than page_sz? > > > > determining the mapped main stack area is i think non-trivial to do in > > > > userspace (requires parsing /proc/self/maps or similar). > > > > > > Because of PROT_GROWSDOWN, the kernel adjusts the start of the range > > > down automatically. It is potentially problematic if the top of the > > > stack is more than a page away and you want the whole stack coloured. I > > > haven't run a test but my reading of the kernel code is that the stack > > > vma would be split in this scenario, so the range beyond sp+page_sz > > > won't have PROT_MTE set. > > > > > > My assumption is that if you do this during program start, the stack is > > > smaller than a page. Alternatively, could we use argv or envp to > > > determine the top of the user stack (the bottom is taken care of by the > > > kernel)? > > > > I don't think you can easily know when the stack ends, but perhaps it > > doesn't matter. > > > > From memory, the initial stack looks like: > > > > argv/env strings > > AT_NULL > > auxv > > NULL > > env > > NULL > > argv > > argc <--- sp > > > > If we don't care about tagging the strings correctly, we could step to > > the end of auxv and tag down from there. > > > > If we do care about tagging the strings, there's probably no good way > > to find the end of the string area, other than looking up sp in > > /proc/self/maps. I'm not sure we should trust all past and future > > kernels to spit out the strings in a predictable order. > > I don't think we care about tagging whatever the kernel places on the > stack since the argv/envp pointers are untagged. An mprotect(PROT_MTE) > may or may not cover the environment but it shouldn't matter as the > kernel clears the tags on the corresponding pages anyway. We have no match-all tag, right? So we do rely on the tags being cleared for the initial stack contents so that using untagged pointers to access it works. > AFAIK stack tagging works by colouring a stack frame on function entry > and clearing the tags on return. We would only hit a problem if the > function issuing mprotect(sp, PROT_MTE) on and its callers already > assumed a PROT_MTE stack. Without PROT_MTE, an STG would be > write-ignore, so subsequently turning it on would lead to a mismatch > between the pointer and the allocation tags. > > So PROT_MTE turning on should happen very early in the user process > startup code before any code with stack tagging enabled. Whether you > reach the top of the stack with such mprotect() doesn't really matter > since up to that point there should not be any use of stack tagging. If > that's not possible, for example the glibc code setting up the stack was > compiled to stack tagging itself, the kernel would have to enable it > when the user process starts. However, I'd only do this based on some > ELF note. Sounds fair. This early on, the process shouldn't be exposed to arbitrary, untrusted data. So it's probably not a problem that tagging isn't turned on right from the start. Cheers ---Dave