From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel Date: Thu, 23 May 2019 08:34:25 +0100 Message-ID: <20190523073425.GA43379@MBP.local> References: <20190517144931.GA56186@arrakis.emea.arm.com> <20190521182932.sm4vxweuwo5ermyd@mbp> <201905211633.6C0BF0C2@keescook> <20190522101110.m2stmpaj7seezveq@mbp> <20190522163527.rnnc6t4tll7tk5zw@mbp> <201905221316.865581CF@keescook> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: enh Cc: Evgenii Stepanov , Kees Cook , Andrey Konovalov , Khalid Aziz , Linux ARM , Linux Memory Management List , LKML , amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-rdma@vger.kernel.org, linux-media@vger.kernel.org, kvm@vger.kernel.org, "open list:KERNEL SELFTEST FRAMEWORK" , Vincenzo Frascino , Will Deacon , Mark Rutland , Andrew Morton , Greg Kroah-Hartman , Yishai Hadas List-Id: linux-rdma@vger.kernel.org On Wed, May 22, 2019 at 04:09:31PM -0700, enh wrote: > On Wed, May 22, 2019 at 4:03 PM Evgenii Stepanov wrote: > > 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: > > > > 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). > > no, i meant "they're encoded in a space that was previously no-ops, so > running on MTE code on old hardware doesn't cause SIGILL". It does result in SIGILL, there wasn't enough encoding left in the NOP space for old/current CPU implementations (in hindsight, we should have reserved a bigger NOP space). As Evgenii said, the libc needs to be careful when tagging the heap as it would cause a SIGILL if the HWCAP_MTE is not set. The standard application doesn't need to be recompiled as it would not issue MTE colouring instructions, just standard LDR/STR. Stack tagging is problematic if you want to colour each frame individually, the function prologue would need the non-NOP MTE instructions. The best we can do here is just having the (thread) stacks of different colours. -- Catalin