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=-7.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,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 C93D3C282D8 for ; Sat, 2 Feb 2019 03:42:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7D76821872 for ; Sat, 2 Feb 2019 03:42:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="F3oldL6D" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727539AbfBBDmt (ORCPT ); Fri, 1 Feb 2019 22:42:49 -0500 Received: from mail-vs1-f67.google.com ([209.85.217.67]:44266 "EHLO mail-vs1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726246AbfBBDmt (ORCPT ); Fri, 1 Feb 2019 22:42:49 -0500 Received: by mail-vs1-f67.google.com with SMTP id u11so5463760vsp.11; Fri, 01 Feb 2019 19:42:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=qgcb8hLqRxiJqamjuHuYd03tKr2bYHzyCOgKFdWGaXw=; b=F3oldL6DlTy1qPp6Uj12iGzuHnXDmyVOVd1GzwX/E1SZK2k0Li84gdWkivoLPU1Qqx IAwUKtVgnaTQwI8Qt+oRlgOHGOarAi67p3vmsDwKpOkhZzGp2Z6H8K1CGy7872Lfi8uY dj8yWAacXwhPjL4LoGvZl8LAesKxt/kFVtazcq2r0hj1mw2d3OqNjY0a5IZg/Wq9Kjal 4HAYGPCGSoMyw4JzjHNww9KqNMCIFwSdEz1tOQuvOKBTWLsTYfZ3chBljUI3BWv0QFa/ 6l6boNrYZVwcL2JdBqOoxwJlMw2TjkyFzhrS5QpVohovsDs2OB6ESmaE6E2AgD+/MiEf 22Eg== X-Google-DKIM-Signature: v=1; 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=qgcb8hLqRxiJqamjuHuYd03tKr2bYHzyCOgKFdWGaXw=; b=UsJBC6rPCItIGrc/U3OcNhJ/0lqmks5clx7yj0tloWDRT+aMW/d99xV7A/vQQB1bSv 8xGJLViwqxsQaiAIKIuBVmBJJqhcBibyC5K10hk2aQ90a0+vNZiv4Djk87khZ3VbbirG 024MozHMc6XnyCVBiw3tty0kSmcCpILsdNOpfQxJJ0gvkATj4spKRE92eGeSUdngKAf2 apGoclpqxXlzV0sbHL7/4euTde1DuoggxzRkxeAIBKayW6GlrnzN2aPje9/eScMXyZJV aq9bnJyinwRhDWgRycBzUWy6zQqnOULcnbveGE1m9ikLOBnI4o39ljr86mfo9xlFmv3p wYfA== X-Gm-Message-State: AJcUukciErcWQo9QDZs1DiHwNOrvJE/NTl9mB1lHlH6RW/OCZtgBRnfN c5NEn7sxKqz1psFHoFb/42sBoLzzQFhUfVvIk3s= X-Google-Smtp-Source: ALg8bN4NgKZEKqaVIKM/IvB/NOoYSAslViWlpWdovWtfV7XV9cdQP+YfcE8H4BUmBCQVfPR0OL6R0CEI1DI3BWpD3NY= X-Received: by 2002:a67:7614:: with SMTP id r20mr17715027vsc.89.1549078967524; Fri, 01 Feb 2019 19:42:47 -0800 (PST) MIME-Version: 1.0 References: <20190122155724.27557-1-joe.lawrence@redhat.com> <20190122155724.27557-2-joe.lawrence@redhat.com> <20190202011455.GN26056@350D> In-Reply-To: <20190202011455.GN26056@350D> From: Balbir Singh Date: Sat, 2 Feb 2019 14:42:35 +1100 Message-ID: Subject: Re: [PATCH 1/4] powerpc/64s: Clear on-stack exception marker upon exception return To: Joe Lawrence Cc: "linux-kernel@vger.kernel.org" , "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" , live-patching@vger.kernel.org, Jiri Kosina , Josh Poimboeuf , Michael Ellerman , Nicolai Stange , Torsten Duwe Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Feb 2, 2019 at 12:14 PM Balbir Singh wrote: > > On Tue, Jan 22, 2019 at 10:57:21AM -0500, 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 > > --- > > arch/powerpc/Kconfig | 2 +- > > arch/powerpc/kernel/entry_64.S | 7 +++++++ > > 2 files changed, 8 insertions(+), 1 deletion(-) > > > > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig > > index 2890d36eb531..73bf87b1d274 100644 > > --- a/arch/powerpc/Kconfig > > +++ b/arch/powerpc/Kconfig > > @@ -220,7 +220,7 @@ config PPC > > select HAVE_PERF_USER_STACK_DUMP > > select HAVE_RCU_TABLE_FREE if SMP > > select HAVE_REGS_AND_STACK_ACCESS_API > > - select HAVE_RELIABLE_STACKTRACE if PPC64 && CPU_LITTLE_ENDIAN > > + select HAVE_RELIABLE_STACKTRACE if PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN > > select HAVE_SYSCALL_TRACEPOINTS > > select HAVE_VIRT_CPU_ACCOUNTING > > select HAVE_IRQ_TIME_ACCOUNTING > > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S > > index 435927f549c4..a2c168b395d2 100644 > > --- a/arch/powerpc/kernel/entry_64.S > > +++ b/arch/powerpc/kernel/entry_64.S > > @@ -1002,6 +1002,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR) > > ld r2,_NIP(r1) > > mtspr SPRN_SRR0,r2 > > > > + /* > > + * Leaving a stale exception_marker on the stack can confuse > > + * the reliable stack unwinder later on. Clear it. > > + */ > > + li r2,0 > > + std r2,STACK_FRAME_OVERHEAD-16(r1) > > + > > Could you please double check, r4 is already 0 at this point > IIUC. So the change might be a simple > > std r4,STACK_FRAME_OVERHEAD-16(r1) > r4 is not 0, sorry for the noise Balbir