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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 69BACC169C4 for ; Thu, 31 Jan 2019 05:47:41 +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 EDE952087F for ; Thu, 31 Jan 2019 05:47:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EDE952087F 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 43qq565rn9zDqWr for ; Thu, 31 Jan 2019 16:47:38 +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 43qq3L2vHxzDqP3 for ; Thu, 31 Jan 2019 16:46:06 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPSA id 43qq3K6r8jz9sDX; Thu, 31 Jan 2019 16:46:05 +1100 (AEDT) From: Michael Ellerman To: Nicolai Stange Subject: Re: [PATCH 1/4] powerpc/64s: Clear on-stack exception marker upon exception return In-Reply-To: <87a7jirrn6.fsf@suse.de> References: <20190122155724.27557-1-joe.lawrence@redhat.com> <20190122155724.27557-2-joe.lawrence@redhat.com> <87tvhqpbzd.fsf@concordia.ellerman.id.au> <87a7jirrn6.fsf@suse.de> Date: Thu, 31 Jan 2019 16:46:05 +1100 Message-ID: <87zhrhnzvm.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain 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: Joe Lawrence , Nicolai Stange , Jiri Kosina , linux-kernel@vger.kernel.org, Torsten Duwe , Josh Poimboeuf , live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Nicolai Stange writes: > 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. No worries, I split it out: commit a50d3250d7ae34c561177a1f9cfb79816fcbcff1 Author: Nicolai Stange AuthorDate: Thu Jan 31 16:41:50 2019 +1100 Commit: Michael Ellerman CommitDate: Thu Jan 31 16:43:29 2019 +1100 powerpc/64s: Make reliable stacktrace dependency clearer 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. Signed-off-by: Nicolai Stange Signed-off-by: Joe Lawrence [mpe: Split out of larger patch] Signed-off-by: Michael Ellerman 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 cheers