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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 7A830C433E4 for ; Mon, 13 Jul 2020 09:48:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 60C362077D for ; Mon, 13 Jul 2020 09:48:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729035AbgGMJsr (ORCPT ); Mon, 13 Jul 2020 05:48:47 -0400 Received: from mga05.intel.com ([192.55.52.43]:4743 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726571AbgGMJsq (ORCPT ); Mon, 13 Jul 2020 05:48:46 -0400 IronPort-SDR: E2+0JxC4esRGlKyhw+7mMBIrHOaV3huPf4KeZRhw4ykNvks9Uv+1gQpaJkVfHe1v7Pvw0WSB6j p4IbwOt1C8bQ== X-IronPort-AV: E=McAfee;i="6000,8403,9680"; a="233441956" X-IronPort-AV: E=Sophos;i="5.75,347,1589266800"; d="scan'208";a="233441956" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Jul 2020 02:48:38 -0700 IronPort-SDR: L14eLNOG7+vwt/gUze0GN5sjUdM/MU8jqTcbQ9b3v71MLCPCZqZytwKXRulOQNxowZGOBGFJUx qnqpy+9+rocg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.75,347,1589266800"; d="scan'208";a="268327366" Received: from linux.intel.com ([10.54.29.200]) by fmsmga007.fm.intel.com with ESMTP; 13 Jul 2020 02:48:38 -0700 Received: from [10.249.229.49] (abudanko-mobl.ccr.corp.intel.com [10.249.229.49]) by linux.intel.com (Postfix) with ESMTP id 9AA74580806; Mon, 13 Jul 2020 02:48:27 -0700 (PDT) Subject: Re: [PATCH v8 00/12] Introduce CAP_PERFMON to secure system performance monitoring and observability To: Arnaldo Carvalho de Melo Cc: Ravi Bangoria , Peter Zijlstra , Alexei Starovoitov , Ingo Molnar , James Morris , Namhyung Kim , Serge Hallyn , Jiri Olsa , Song Liu , Andi Kleen , Stephane Eranian , Igor Lubashev , Thomas Gleixner , linux-kernel , "linux-security-module@vger.kernel.org" , "selinux@vger.kernel.org" , "intel-gfx@lists.freedesktop.org" , "linux-doc@vger.kernel.org" , linux-man@vger.kernel.org References: <76718dc6-5483-5e2e-85b8-64e70306ee1f@linux.ibm.com> <7776fa40-6c65-2aa6-1322-eb3a01201000@linux.intel.com> <20200710170911.GD7487@kernel.org> From: Alexey Budankov Organization: Intel Corp. Message-ID: <0d2e2306-22b2-a730-dc3f-edb3538b6561@linux.intel.com> Date: Mon, 13 Jul 2020 12:48:25 +0300 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200710170911.GD7487@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 10.07.2020 20:09, Arnaldo Carvalho de Melo wrote: > Em Fri, Jul 10, 2020 at 05:30:50PM +0300, Alexey Budankov escreveu: >> On 10.07.2020 16:31, Ravi Bangoria wrote: >>>> Currently access to perf_events, i915_perf and other performance >>>> monitoring and observability subsystems of the kernel is open only for >>>> a privileged process [1] with CAP_SYS_ADMIN capability enabled in the >>>> process effective set [2]. > >>>> This patch set introduces CAP_PERFMON capability designed to secure >>>> system performance monitoring and observability operations so that >>>> CAP_PERFMON would assist CAP_SYS_ADMIN capability in its governing role >>>> for performance monitoring and observability subsystems of the kernel. > >>> I'm seeing an issue with CAP_PERFMON when I try to record data for a >>> specific target. I don't know whether this is sort of a regression or >>> an expected behavior. > >> Thanks for reporting and root causing this case. The behavior looks like >> kind of expected since currently CAP_PERFMON takes over the related part >> of CAP_SYS_ADMIN credentials only. Actually Perf security docs [1] say >> that access control is also subject to CAP_SYS_PTRACE credentials. > > I think that stating that in the error message would be helpful, after > all, who reads docs? 8-) At least those who write it :D ... > > I.e., this: > > $ ./perf stat ls >   Error: >   Access to performance monitoring and observability operations is limited. > $ > > Could become: > > $ ./perf stat ls >   Error: >   Access to performance monitoring and observability operations is limited. > Right now only CAP_PERFMON is granted, you may need CAP_SYS_PTRACE. > $ It would better provide reference to perf security docs in the tool output. Looks like extending ptrace_may_access() check for perf_events with CAP_PERFMON makes monitoring simpler and even more secure to use since Perf tool need not to start/stop/single-step and read/write registers and memory and so on like a debugger or strace-like tool. What do you think? Alexei > > - Arnaldo > >> CAP_PERFMON could be used to extend and substitute ptrace_may_access() >> check in perf_events subsystem to simplify user experience at least in >> this specific case. >> >> Alexei >> >> [1] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html >> >>> >>> Without setting CAP_PERFMON: >>> >>>   $ getcap ./perf >>>   $ ./perf stat -a ls >>>     Error: >>>     Access to performance monitoring and observability operations is limited. >>>   $ ./perf stat ls >>>     Performance counter stats for 'ls': >>>                     2.06 msec task-clock:u              #    0.418 CPUs utilized >>>                     0      context-switches:u        #    0.000 K/sec >>>                     0      cpu-migrations:u          #    0.000 K/sec >>> >>> With CAP_PERFMON: >>> >>>   $ getcap ./perf >>>     ./perf = cap_perfmon+ep >>>   $ ./perf stat -a ls >>>     Performance counter stats for 'system wide': >>>                   142.42 msec cpu-clock                 #   25.062 CPUs utilized >>>                   182      context-switches          #    0.001 M/sec >>>                    48      cpu-migrations            #    0.337 K/sec >>>   $ ./perf stat ls >>>     Error: >>>     Access to performance monitoring and observability operations is limited. >>> >>> Am I missing something silly? >>> >>> Analysis: >>> --------- >>> A bit more analysis lead me to below kernel code fs/exec.c: >>> >>>   begin_new_exec() >>>   { >>>         ... >>>         if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP || >>>             !(uid_eq(current_euid(), current_uid()) && >>>               gid_eq(current_egid(), current_gid()))) >>>                 set_dumpable(current->mm, suid_dumpable); >>>         else >>>                 set_dumpable(current->mm, SUID_DUMP_USER); >>> >>>         ... >>>         commit_creds(bprm->cred); >>>   } >>> >>> When I execute './perf stat ls', it's going into else condition and thus sets >>> dumpable flag as SUID_DUMP_USER. Then in commit_creds(): >>> >>>   int commit_creds(struct cred *new) >>>   { >>>         ... >>>         /* dumpability changes */ >>>         if (... >>>             !cred_cap_issubset(old, new)) { >>>                 if (task->mm) >>>                         set_dumpable(task->mm, suid_dumpable); >>>   } >>> >>> !cred_cap_issubset(old, new) fails for perf without any capability and thus >>> it doesn't execute set_dumpable(). Whereas that condition passes for perf >>> with CAP_PERFMON and thus it overwrites old value (SUID_DUMP_USER) with >>> suid_dumpable in mm_flags. On an Ubuntu, suid_dumpable default value is >>> SUID_DUMP_ROOT. On Fedora, it's SUID_DUMP_DISABLE. (/proc/sys/fs/suid_dumpable). >>> >>> Now while opening an event: >>> >>>   perf_event_open() >>>     ptrace_may_access() >>>       __ptrace_may_access() { >>>                 ... >>>                 if (mm && >>>                     ((get_dumpable(mm) != SUID_DUMP_USER) && >>>                      !ptrace_has_cap(cred, mm->user_ns, mode))) >>>                     return -EPERM; >>>       } >>> >>> This if condition passes for perf with CAP_PERFMON and thus it returns -EPERM. >>> But it fails for perf without CAP_PERFMON and thus it goes ahead and returns >>> success. So opening an event fails when perf has CAP_PREFMON and tries to open >>> process specific event as normal user. >>> >>> Workarounds: >>> ------------ >>> Based on above analysis, I found couple of workarounds (examples are on >>> Ubuntu 18.04.4 powerpc): >>> >>> Workaround1: >>> Setting SUID_DUMP_USER as default (in /proc/sys/fs/suid_dumpable) solves the >>> issue. >>> >>>   # echo 1 > /proc/sys/fs/suid_dumpable >>>   $ getcap ./perf >>>     ./perf = cap_perfmon+ep >>>   $ ./perf stat ls >>>     Performance counter stats for 'ls': >>>                     1.47 msec task-clock                #    0.806 CPUs utilized >>>                     0      context-switches          #    0.000 K/sec >>>                     0      cpu-migrations            #    0.000 K/sec >>> >>> Workaround2: >>> Using CAP_SYS_PTRACE along with CAP_PERFMON solves the issue. >>> >>>   $ cat /proc/sys/fs/suid_dumpable >>>     2 >>>   # setcap "cap_perfmon,cap_sys_ptrace=ep" ./perf >>>   $ ./perf stat ls >>>     Performance counter stats for 'ls': >>>                     1.41 msec task-clock                #    0.826 CPUs utilized >>>                     0      context-switches          #    0.000 K/sec >>>                     0      cpu-migrations            #    0.000 K/sec >>> >>> Workaround3: >>> Adding CAP_PERFMON to parent of perf (/bin/bash) also solves the issue. >>> >>>   $ cat /proc/sys/fs/suid_dumpable >>>     2 >>>   # setcap "cap_perfmon=ep" /bin/bash >>>   # setcap "cap_perfmon=ep" ./perf >>>   $ bash >>>   $ ./perf stat ls >>>     Performance counter stats for 'ls': >>>                     1.47 msec task-clock                #    0.806 CPUs utilized >>>                     0      context-switches          #    0.000 K/sec >>>                     0      cpu-migrations            #    0.000 K/sec >>> >>> - Ravi >