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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 13E5CC433E7 for ; Fri, 28 Aug 2020 09:56:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E40F9208FE for ; Fri, 28 Aug 2020 09:56:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728989AbgH1J4t (ORCPT ); Fri, 28 Aug 2020 05:56:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:36244 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728016AbgH1J4r (ORCPT ); Fri, 28 Aug 2020 05:56:47 -0400 Received: from gaia (unknown [46.69.195.127]) (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 3CAEB208D5; Fri, 28 Aug 2020 09:56:44 +0000 (UTC) Date: Fri, 28 Aug 2020 10:56:41 +0100 From: Catalin Marinas To: Evgenii Stepanov Cc: Andrey Konovalov , Vincenzo Frascino , Dmitry Vyukov , kasan-dev , Andrey Ryabinin , Alexander Potapenko , Marco Elver , Elena Petrova , Branislav Rankov , Kevin Brodsky , Will Deacon , Andrew Morton , Linux ARM , Linux Memory Management List , LKML Subject: Re: [PATCH 21/35] arm64: mte: Add in-kernel tag fault handler Message-ID: <20200828095641.GD3169@gaia> References: <20200827095429.GC29264@gaia> <20200827131045.GM29264@gaia> <20200827145642.GO29264@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) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 27, 2020 at 12:14:26PM -0700, Evgenii Stepanov wrote: > On Thu, Aug 27, 2020 at 7:56 AM Catalin Marinas wrote: > > On Thu, Aug 27, 2020 at 03:34:42PM +0200, Andrey Konovalov wrote: > > > On Thu, Aug 27, 2020 at 3:10 PM Catalin Marinas wrote: > > > > On Thu, Aug 27, 2020 at 02:31:23PM +0200, Andrey Konovalov wrote: > > > > > On Thu, Aug 27, 2020 at 11:54 AM Catalin Marinas > > > > > wrote: > > > > > > On Fri, Aug 14, 2020 at 07:27:03PM +0200, Andrey Konovalov wrote: > > > > > > > +static int do_tag_recovery(unsigned long addr, unsigned int esr, > > > > > > > + struct pt_regs *regs) > > > > > > > +{ > > > > > > > + report_tag_fault(addr, esr, regs); > > > > > > > + > > > > > > > + /* Skip over the faulting instruction and continue: */ > > > > > > > + arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE); > > > > > > > > > > > > Ooooh, do we expect the kernel to still behave correctly after this? I > > > > > > thought the recovery means disabling tag checking altogether and > > > > > > restarting the instruction rather than skipping over it. > > [...] > > > > > Can we disable MTE, reexecute the instruction, and then reenable MTE, > > > > > or something like that? > > > > > > > > If you want to preserve the MTE enabled, you could single-step the > > > > instruction or execute it out of line, though it's a bit more convoluted > > > > (we have a similar mechanism for kprobes/uprobes). > > > > > > > > Another option would be to attempt to set the matching tag in memory, > > > > under the assumption that it is writable (if it's not, maybe it's fine > > > > to panic). Not sure how this interacts with the slub allocator since, > > > > presumably, the logical tag in the pointer is wrong rather than the > > > > allocation one. > > > > > > > > Yet another option would be to change the tag in the register and > > > > re-execute but this may confuse the compiler. > > > > > > Which one of these would be simpler to implement? > > > > Either 2 or 3 would be simpler (re-tag the memory location or the > > pointer) with the caveats I mentioned. Also, does the slab allocator > > need to touch the memory on free with a tagged pointer? Otherwise slab > > may hit an MTE fault itself. > > Changing the memory tag can cause faults in other threads, and that > could be very confusing. It could indeed trigger a chain of faults. It's not even other threads, it could be the same thread in a different function. > Probably the safest thing is to retag the register, single step and > then retag it back, but be careful with the instructions that change > the address register (like ldr x0, [x0]). This gets complicated if you have to parse the opcode. If you can single-step, just set PSTATE.TCO for the instruction. But the single-step machinery gets more complicated, probably interacts badly with kprobes. I think the best option is to disable the MTE checks in TCF on an _unhandled_ kernel fault, report and continue. For the KASAN tests, add accessors similar to get_user/put_user which are able to handle the fault and return an error. Such accessors, since they have a fixup handler, would not lead to the MTE checks being disabled. -- Catalin