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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=083fICcLHdIZzVNufU25syYhE84WJwTKtiHOZtf8SA0=; b=GPA4VLRbnnDN+NaXBW+aBvGZ+jiB+7f1lSd7akOOUzm0psJ+3En9Co0YQ4RiLMFwtF K3dSV3vHnwJTOXmJBf0w9p5FgDbAmAGPh1XSLe0blewWcmmaP77Lt+PRdngBExhi48ef eOXkhZxDCeWXmYleA3dI7Rm0IEna/FZj5BcD1I1E9ZBFYDo2GZ8HqLh5XDP11ZuT6A3O Kdmw0ZMnNDkwcGmgTcZS1VYAb0ZH6qVyfF6EkiQSgdCCle1SYg6dC0iZZEbdetd52ebr q7IvG0F1uvgJKJ71b7WFPevkdh1uV7uAWYgrYONrSD3KYGXvWhCi8ISsMTqpufuWEyar wfjg== X-Gm-Message-State: APjAAAUMo0V+1vaPlEJWg3x640q9nj/CqLS64DGXHCQptQIwUkauR/ZT QBctHEepONduh3TsbWhCNMjnZa9jKHprEwGxOsvTwQ== X-Google-Smtp-Source: APXvYqxPKtiNPawqkCkuPCNw0GStvO0zt/hmYPvT2nokhB6CEYZVKos9gwheEkiU5Yy2rN1PLFcfrDl2WDUSCoUDbnM= X-Received: by 2002:a1f:4ec6:: with SMTP id c189mr128107vkb.17.1558566229772; Wed, 22 May 2019 16:03:49 -0700 (PDT) MIME-Version: 1.0 References: <20190517144931.GA56186@arrakis.emea.arm.com> <20190521182932.sm4vxweuwo5ermyd@mbp> <201905211633.6C0BF0C2@keescook> <20190522101110.m2stmpaj7seezveq@mbp> <20190522163527.rnnc6t4tll7tk5zw@mbp> <201905221316.865581CF@keescook> In-Reply-To: <201905221316.865581CF@keescook> From: Evgenii Stepanov Date: Wed, 22 May 2019 16:03:36 -0700 Message-ID: Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel To: Kees Cook X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190522_160352_101924_CD9C13A4 X-CRM114-Status: GOOD ( 29.96 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , kvm@vger.kernel.org, Szabolcs Nagy , Catalin Marinas , Will Deacon , dri-devel@lists.freedesktop.org, Linux Memory Management List , Khalid Aziz , "open list:KERNEL SELFTEST FRAMEWORK" , Felix Kuehling , Vincenzo Frascino , Jacob Bramley , Leon Romanovsky , linux-rdma@vger.kernel.org, amd-gfx@lists.freedesktop.org, Dmitry Vyukov , Dave Martin , linux-media@vger.kernel.org, Kevin Brodsky , Ruben Ayrapetyan , Andrey Konovalov , Ramana Radhakrishnan , Alex Williamson , Mauro Carvalho Chehab , Linux ARM , Kostya Serebryany , Greg Kroah-Hartman , Yishai Hadas , LKML , Jens Wiklander , Lee Smith , Alexander Deucher , Andrew Morton , enh , Robin Murphy , Christian Koenig , Luc Van Oostenryck Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, May 22, 2019 at 1:47 PM Kees Cook wrote: > > On Wed, May 22, 2019 at 05:35:27PM +0100, Catalin Marinas wrote: > > The two hard requirements I have for supporting any new hardware feature > > in Linux are (1) a single kernel image binary continues to run on old > > hardware while making use of the new feature if available and (2) old > > user space continues to run on new hardware while new user space can > > take advantage of the new feature. > > Agreed! And I think the series meets these requirements, yes? > > > For MTE, we just can't enable it by default since there are applications > > who use the top byte of a pointer and expect it to be ignored rather > > than failing with a mismatched tag. Just think of a hwasan compiled > > binary where TBI is expected to work and you try to run it with MTE > > turned on. > > Ah! Okay, here's the use-case I wasn't thinking of: the concern is TBI > conflicting with MTE. And anything that starts using TBI suddenly can't > run in the future because it's being interpreted as MTE bits? (Is that > the ABI concern? I feel like we got into the weeds about ioctl()s and > one-off bugs...) > > So there needs to be some way to let the kernel know which of three > things it should be doing: > 1- leaving userspace addresses as-is (present) > 2- wiping the top bits before using (this series) > 3- wiping the top bits for most things, but retaining them for MTE as > needed (the future) > > I expect MTE to be the "default" in the future. Once a system's libc has > grown support for it, everything will be trying to use MTE. TBI will be > the special case (but TBI is effectively a prerequisite). > > AFAICT, the only difference I see between 2 and 3 will be the tag handling > in usercopy (all other places will continue to ignore the top bits). Is > that accurate? > > Is "1" a per-process state we want to keep? (I assume not, but rather it > is available via no TBI/MTE CONFIG or a boot-time option, if at all?) > > To choose between "2" and "3", it seems we need a per-process flag to > opt into TBI (and out of MTE). For userspace, how would a future binary > choose TBI over MTE? If it's a library issue, we can't use an ELF bit, > since the choice may be "late" after ELF load (this implies the need > for a prctl().) If it's binary-only ("built with HWKASan") then an ELF > bit seems sufficient. And without the marking, I'd expect the kernel to > enforce MTE when there are high bits. > > > I would also expect the C library or dynamic loader to check for the > > presence of a HWCAP_MTE bit before starting to tag memory allocations, > > otherwise it would get SIGILL on the first MTE instruction it tries to > > execute. > > I've got the same question as Elliot: aren't MTE instructions just NOP > to older CPUs? I.e. if the CPU (or kernel) don't support it, it just > gets entirely ignored: checking is only needed to satisfy curiosity > or behavioral expectations. MTE instructions are not NOP. Most of them have side effects (changing register values, zeroing memory). This only matters for stack tagging, though. Heap tagging is a runtime decision in the allocator. If an image needs to run on old hardware, it will have to do heap tagging only. > To me, the conflict seems to be using TBI in the face of expecting MTE to > be the default state of the future. (But the internal changes needed > for TBI -- this series -- is a prereq for MTE.) > > -- > Kees Cook _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel