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 autolearn=unavailable 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 E0361C169C4 for ; Fri, 8 Feb 2019 14:04:53 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6436B20857 for ; Fri, 8 Feb 2019 14:04:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6436B20857 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 43wxl71rv9zDqY3 for ; Sat, 9 Feb 2019 01:04:51 +1100 (AEDT) Received: from ozlabs.org (bilbo.ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 43wwMG3ld9zDqGn for ; Sat, 9 Feb 2019 00:02:34 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au 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 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) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jiri Kosina , Torsten Duwe , Nicolai Stange , Josh Poimboeuf Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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