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=-12.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,HTML_MESSAGE,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 9A3BFC48BE5 for ; Tue, 15 Jun 2021 11:34:34 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 04F8161455 for ; Tue, 15 Jun 2021 11:34:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 04F8161455 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=eldorado.org.br Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:52436 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lt7Ky-0001cJ-Qd for qemu-devel@archiver.kernel.org; Tue, 15 Jun 2021 07:34:32 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51900) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lt7J9-0000bO-Sx; Tue, 15 Jun 2021 07:32:39 -0400 Received: from [201.28.113.2] (port=53513 helo=outlook.eldorado.org.br) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lt7J7-0006ax-5v; Tue, 15 Jun 2021 07:32:39 -0400 Received: from power9a ([10.10.71.235]) by outlook.eldorado.org.br with Microsoft SMTPSVC(8.5.9600.16384); Tue, 15 Jun 2021 08:32:33 -0300 Received: from [127.0.0.1] (unknown [10.10.71.235]) by power9a (Postfix) with ESMTPS id 0FFDA800144; Tue, 15 Jun 2021 08:32:33 -0300 (-03) Subject: Re: [RFC PATCH v2 2/2] target/ppc: make gdb able to translate priviledged addresses To: Richard Henderson , qemu-devel@nongnu.org References: <20210614191630.101304-1-bruno.larsen@eldorado.org.br> <20210614191630.101304-2-bruno.larsen@eldorado.org.br> From: Bruno Piazera Larsen Message-ID: <1c27c473-be10-41cf-d633-bcd838fed78e@eldorado.org.br> Date: Tue, 15 Jun 2021 08:32:32 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/alternative; boundary="------------786AE4A034BC2D76769D6CA8" Content-Language: en-US X-OriginalArrivalTime: 15 Jun 2021 11:32:33.0377 (UTC) FILETIME=[221C6510:01D761DA] X-Host-Lookup-Failed: Reverse DNS lookup failed for 201.28.113.2 (failed) Received-SPF: pass client-ip=201.28.113.2; envelope-from=bruno.larsen@eldorado.org.br; helo=outlook.eldorado.org.br X-Spam_score_int: -8 X-Spam_score: -0.9 X-Spam_bar: / X-Spam_report: (-0.9 / 5.0 requ) BAYES_00=-1.9, HTML_MESSAGE=0.001, NICE_REPLY_A=-0.095, PDS_HP_HELO_NORDNS=0.308, RDNS_NONE=0.793, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: farosas@linux.ibm.com, luis.pires@eldorado.org.br, Greg Kurz , lucas.araujo@eldorado.org.br, fernando.valle@eldorado.org.br, qemu-ppc@nongnu.org, matheus.ferst@eldorado.org.br, david@gibson.dropbear.id.au Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is a multi-part message in MIME format. --------------786AE4A034BC2D76769D6CA8 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit On 14/06/2021 19:37, Richard Henderson wrote: > On 6/14/21 12:16 PM, Bruno Larsen (billionai) wrote: >> This patch changes ppc_cpu_get_phys_page_debug so that it is now >> able to translate both, priviledged and real mode addresses >> independently of whether the CPU executing it has those permissions >> >> This was mentioned by Fabiano as something that would be very useful to >> help with debugging, but could possibly constitute a security issue if >> that debug function can be called in some way by prodution code. the >> solution was implemented such that it would be trivial to wrap it around >> ifdefs for building only with --enable-debug, for instance, but we are >> not sure this is the best approach, hence why it is an RFC. >> >> Suggested-by: Fabiano Rosas >> Signed-off-by: Bruno Larsen (billionai) >> --- >>   target/ppc/mmu_helper.c | 23 +++++++++++++++++++++++ >>   1 file changed, 23 insertions(+) > > I think the first part is unnecessary.  Either the cpu is in > supervisor mode or it isn't, and gdb should use the correct address > space.  If you really want to force supervisor lookup from a guest > that is paused in usermode, I suppose you could force MSR.PR=1 while > you're performing the access and set it back afterward. I don't see why GDB should not be able to see supervisor level addresses just because the CPU can't. when debugging, we wanna see exactly what QEMU sees, not what the guest sees, right? Now, if this is changing more than just privilege level, I agree there is a problem, but I wouldn't think it is the case... > > I think the second part is actively wrong -- real-mode address lookup > will (for the most part) always succeed.  Moreover, the gdb user will > have no idea that you've silently changed addressing methods. I disagree. Real-mode address will mostly fail, since during the boot process Linux kernels set the MMU to use only virtual addresses, so real mode addresses only work when debugging the firmware or the early setup of the kernel. After that, GDB can basically only see virtual addresses. Maybe there is a better way to handle this by having GDB warn the user that the CPU can not decode the address in it's current state, but I do think it is a good tool to have, as it would've made debugging the first RFC on this topic a bit easier, and farosas was actively complaining that isn't a feature yet. > > r~ -- Bruno Piazera Larsen Instituto de Pesquisas ELDORADO Departamento Computação Embarcada Analista de Software Trainee Aviso Legal - Disclaimer --------------786AE4A034BC2D76769D6CA8 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit
On 14/06/2021 19:37, Richard Henderson wrote:
On 6/14/21 12:16 PM, Bruno Larsen (billionai) wrote:
This patch changes ppc_cpu_get_phys_page_debug so that it is now
able to translate both, priviledged and real mode addresses
independently of whether the CPU executing it has those permissions

This was mentioned by Fabiano as something that would be very useful to
help with debugging, but could possibly constitute a security issue if
that debug function can be called in some way by prodution code. the
solution was implemented such that it would be trivial to wrap it around
ifdefs for building only with --enable-debug, for instance, but we are
not sure this is the best approach, hence why it is an RFC.

Suggested-by: Fabiano Rosas<farosas@linux.ibm.com>
Signed-off-by: Bruno Larsen (billionai)<bruno.larsen@eldorado.org.br>
---
  target/ppc/mmu_helper.c | 23 +++++++++++++++++++++++
  1 file changed, 23 insertions(+)

I think the first part is unnecessary.  Either the cpu is in supervisor mode or it isn't, and gdb should use the correct address space.  If you really want to force supervisor lookup from a guest that is paused in usermode, I suppose you could force MSR.PR=1 while you're performing the access and set it back afterward.
I don't see why GDB should not be able to see supervisor level addresses just because the CPU can't. when debugging, we wanna see exactly what QEMU sees, not what the guest sees, right? Now, if this is changing more than just privilege level, I agree there is a problem, but I wouldn't think it is the case...

I think the second part is actively wrong -- real-mode address lookup will (for the most part) always succeed.  Moreover, the gdb user will have no idea that you've silently changed addressing methods.

I disagree. Real-mode address will mostly fail, since during the boot process Linux kernels set the MMU to use only virtual addresses, so real mode addresses only work when debugging the firmware or the early setup of the kernel. After that, GDB can basically only see virtual addresses.

Maybe there is a better way to handle this by having GDB warn the user that the CPU can not decode the address in it's current state, but I do think it is a good tool to have, as it would've made debugging the first RFC on this topic a bit easier, and farosas was actively complaining that isn't a feature yet.


r~
--
Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO
Departamento Computação Embarcada
Analista de Software Trainee
Aviso Legal - Disclaimer
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