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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 17281C282D7 for ; Wed, 30 Jan 2019 17:18:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E2B702087F for ; Wed, 30 Jan 2019 17:18:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732466AbfA3RSK convert rfc822-to-8bit (ORCPT ); Wed, 30 Jan 2019 12:18:10 -0500 Received: from mx2.suse.de ([195.135.220.15]:40404 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726972AbfA3RSJ (ORCPT ); Wed, 30 Jan 2019 12:18:09 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 848FCAD3C; Wed, 30 Jan 2019 17:18:07 +0000 (UTC) From: Nicolai Stange To: Michael Ellerman Cc: Joe Lawrence , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, live-patching@vger.kernel.org, Balbir Singh , Jiri Kosina , Josh Poimboeuf , Nicolai Stange , Torsten Duwe Subject: Re: [PATCH 1/4] powerpc/64s: Clear on-stack exception marker upon exception return References: <20190122155724.27557-1-joe.lawrence@redhat.com> <20190122155724.27557-2-joe.lawrence@redhat.com> <87tvhqpbzd.fsf@concordia.ellerman.id.au> Date: Wed, 30 Jan 2019 18:18:05 +0100 In-Reply-To: <87tvhqpbzd.fsf@concordia.ellerman.id.au> (Michael Ellerman's message of "Wed, 30 Jan 2019 23:27:02 +1100") Message-ID: <87a7jirrn6.fsf@suse.de> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Michael Ellerman writes: > Joe Lawrence writes: >> From: Nicolai Stange >> >> The ppc64 specific implementation of the reliable stacktracer, >> save_stack_trace_tsk_reliable(), bails out and reports an "unreliable >> trace" whenever it finds an exception frame on the stack. Stack frames >> are classified as exception frames if the STACK_FRAME_REGS_MARKER magic, >> as written by exception prologues, is found at a particular location. >> >> However, as observed by Joe Lawrence, it is possible in practice that >> non-exception stack frames can alias with prior exception frames and thus, >> that the reliable stacktracer can find a stale STACK_FRAME_REGS_MARKER on >> the stack. It in turn falsely reports an unreliable stacktrace and blocks >> any live patching transition to finish. Said condition lasts until the >> stack frame is overwritten/initialized by function call or other means. >> >> In principle, we could mitigate this by making the exception frame >> classification condition in save_stack_trace_tsk_reliable() stronger: >> in addition to testing for STACK_FRAME_REGS_MARKER, we could also take into >> account that for all exceptions executing on the kernel stack >> - their stack frames's backlink pointers always match what is saved >> in their pt_regs instance's ->gpr[1] slot and that >> - their exception frame size equals STACK_INT_FRAME_SIZE, a value >> uncommonly large for non-exception frames. >> >> However, while these are currently true, relying on them would make the >> reliable stacktrace implementation more sensitive towards future changes in >> the exception entry code. Note that false negatives, i.e. not detecting >> exception frames, would silently break the live patching consistency model. >> >> Furthermore, certain other places (diagnostic stacktraces, perf, xmon) >> rely on STACK_FRAME_REGS_MARKER as well. >> >> Make the exception exit code clear the on-stack STACK_FRAME_REGS_MARKER >> for those exceptions running on the "normal" kernel stack and returning >> to kernelspace: because the topmost frame is ignored by the reliable stack >> tracer anyway, returns to userspace don't need to take care of clearing >> the marker. >> >> Furthermore, as I don't have the ability to test this on Book 3E or >> 32 bits, limit the change to Book 3S and 64 bits. >> >> Finally, make the HAVE_RELIABLE_STACKTRACE Kconfig option depend on >> PPC_BOOK3S_64 for documentation purposes. Before this patch, it depended >> on PPC64 && CPU_LITTLE_ENDIAN and because CPU_LITTLE_ENDIAN implies >> PPC_BOOK3S_64, there's no functional change here. > > That has nothing to do with the fix and should really be in a separate > patch. > > I can split it when applying. If you don't mind, that would be nice! Or simply drop that chunk... Otherwise, let me know if I shall send a split v2 for this patch [1/4] only. Thanks, Nicolai -- SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)