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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 8A6FEC282CB for ; Fri, 8 Feb 2019 13:02:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 64CB020844 for ; Fri, 8 Feb 2019 13:02:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727592AbfBHNCj (ORCPT ); Fri, 8 Feb 2019 08:02:39 -0500 Received: from ozlabs.org ([203.11.71.1]:39993 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727489AbfBHNCg (ORCPT ); Fri, 8 Feb 2019 08:02:36 -0500 Received: by ozlabs.org (Postfix, from userid 1034) id 43wwMG0vPgz9sNL; Sat, 9 Feb 2019 00:02:33 +1100 (AEDT) X-powerpc-patch-notification: thanks X-powerpc-patch-commit: eddd0b332304d554ad6243942f87c2fcea98c56b X-Patchwork-Hint: ignore In-Reply-To: <20190122155724.27557-2-joe.lawrence@redhat.com> To: Joe Lawrence , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, live-patching@vger.kernel.org From: Michael Ellerman Cc: Nicolai Stange , Jiri Kosina , Josh Poimboeuf , Torsten Duwe Subject: Re: [1/4] powerpc/64s: Clear on-stack exception marker upon exception return Message-Id: <43wwMG0vPgz9sNL@ozlabs.org> Date: Sat, 9 Feb 2019 00:02:33 +1100 (AEDT) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2019-01-22 at 15:57:21 UTC, Joe Lawrence wrote: > 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. > > Fixes: df78d3f61480 ("powerpc/livepatch: Implement reliable stack tracing for the consistency model") > Reported-by: Joe Lawrence > Signed-off-by: Nicolai Stange > Signed-off-by: Joe Lawrence Series applied to powerpc next, thanks. https://git.kernel.org/powerpc/c/eddd0b332304d554ad6243942f87c2fc cheers