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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,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 503F4C433E0 for ; Fri, 10 Jul 2020 13:34:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 30E9620578 for ; Fri, 10 Jul 2020 13:34:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727782AbgGJNeY (ORCPT ); Fri, 10 Jul 2020 09:34:24 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:17662 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726840AbgGJNeX (ORCPT ); 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Fri, 10 Jul 2020 13:32:42 +0000 Received: from d06av23.portsmouth.uk.ibm.com (d06av23.portsmouth.uk.ibm.com [9.149.105.59]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 06ADVPdD59965518 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 10 Jul 2020 13:31:25 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4E7BAA4051; Fri, 10 Jul 2020 13:31:25 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E7A08A404D; Fri, 10 Jul 2020 13:31:21 +0000 (GMT) Received: from [9.199.38.25] (unknown [9.199.38.25]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 10 Jul 2020 13:31:21 +0000 (GMT) From: Ravi Bangoria Subject: Re: [PATCH v8 00/12] Introduce CAP_PERFMON to secure system performance monitoring and observability To: Alexey Budankov Cc: Peter Zijlstra , Arnaldo Carvalho de Melo , 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, Ravi Bangoria References: Message-ID: <76718dc6-5483-5e2e-85b8-64e70306ee1f@linux.ibm.com> Date: Fri, 10 Jul 2020 19:01:21 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-07-10_07:2020-07-10,2020-07-10 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 lowpriorityscore=0 clxscore=1011 priorityscore=1501 mlxlogscore=999 malwarescore=0 adultscore=0 bulkscore=0 suspectscore=0 mlxscore=0 impostorscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2007100093 Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hi Alexey, > 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. 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