From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH 0/2] fix the traced mt-exec deadlock Date: Fri, 03 Mar 2017 12:59:19 -0600 Message-ID: <87d1dyw5iw.fsf@xmission.com> References: <20170213141452.GA30203@redhat.com> <20170224160354.GA845@redhat.com> <87shmv6ufl.fsf@xmission.com> <20170303173326.GA17899@redhat.com> <87tw7axlr0.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <87tw7axlr0.fsf@xmission.com> (Eric W. Biederman's message of "Fri, 03 Mar 2017 12:23:31 -0600") Sender: linux-kernel-owner@vger.kernel.org To: Oleg Nesterov Cc: Andrew Morton , Aleksa Sarai , Andy Lutomirski , Attila Fazekas , Jann Horn , Kees Cook , Michal Hocko , Ulrich Obergfell , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org List-Id: linux-api@vger.kernel.org ebiederm@xmission.com (Eric W. Biederman) writes: > The big lesson for me, and what was not obvious from your change > description is that we are changing the user space visible semantics > of exec+ptrace and that cred_guard_mutex is not at all the problem (as > we always take cred_guard_mutex in a killable or interruptible way). Just to follow up. Because the cred_guard_mutex is fine as is we don't need to move de_thread out from under cred_guard_mutex. We just need to change de_thread to wait until all of the other threads are zombies. Which should remove about half your proposed patch. The other key thing is that knowning it isn't cred_guard_mutex let's us know that this kind of deadlock goes all of the way back to when CLONE_THREAD was merged into the kernel. Insteresingly enough looking at zap_other_threads and notify_count I have found a second bug. When a multi-threaded processes becomes a zombie we don't send the notification to the parent process until the non-leader threads have been reaped. Which means ptrace can mess up sending SIGCHLD to the parent. Now arguably that might be what is desirable but I don't think so. If we aren't ptracing a thread then I don't think we want to delay sending SIGCHLD to the parent. So this whole area of the semantics of a ptrace'd multi-threaded process exiting/exec'ing looks like it needs a thorough going over. Eric From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752450AbdCDACx (ORCPT ); Fri, 3 Mar 2017 19:02:53 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:48539 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752391AbdCDACv (ORCPT ); Fri, 3 Mar 2017 19:02:51 -0500 From: ebiederm@xmission.com (Eric W. Biederman) To: Oleg Nesterov Cc: Andrew Morton , Aleksa Sarai , Andy Lutomirski , Attila Fazekas , Jann Horn , Kees Cook , Michal Hocko , Ulrich Obergfell , linux-kernel@vger.kernel.org, References: <20170213141452.GA30203@redhat.com> <20170224160354.GA845@redhat.com> <87shmv6ufl.fsf@xmission.com> <20170303173326.GA17899@redhat.com> <87tw7axlr0.fsf@xmission.com> Date: Fri, 03 Mar 2017 12:59:19 -0600 In-Reply-To: <87tw7axlr0.fsf@xmission.com> (Eric W. Biederman's message of "Fri, 03 Mar 2017 12:23:31 -0600") Message-ID: <87d1dyw5iw.fsf@xmission.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1cjsV9-0001HH-QH;;;mid=<87d1dyw5iw.fsf@xmission.com>;;;hst=in02.mta.xmission.com;;;ip=67.3.234.240;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/PCnKsWWLnSw87PFFN2lihiemTUlYdk3k= X-SA-Exim-Connect-IP: 67.3.234.240 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.0 TVD_RCVD_IP Message was received from an IP address * 0.0 T_TM2_M_HEADER_IN_MSG BODY: No description available. * 0.8 BAYES_50 BODY: Bayes spam probability is 40 to 60% * [score: 0.4474] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa04 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 T_TooManySym_01 4+ unique symbols in subject X-Spam-DCC: XMission; sa04 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: *;Oleg Nesterov X-Spam-Relay-Country: X-Spam-Timing: total 5543 ms - load_scoreonly_sql: 0.05 (0.0%), signal_user_changed: 8 (0.1%), b_tie_ro: 5 (0.1%), parse: 0.75 (0.0%), extract_message_metadata: 10 (0.2%), get_uri_detail_list: 1.05 (0.0%), tests_pri_-1000: 6 (0.1%), tests_pri_-950: 1.06 (0.0%), tests_pri_-900: 0.90 (0.0%), tests_pri_-400: 19 (0.3%), check_bayes: 18 (0.3%), b_tokenize: 6 (0.1%), b_tok_get_all: 6 (0.1%), b_comp_prob: 1.77 (0.0%), b_tok_touch_all: 2.3 (0.0%), b_finish: 0.65 (0.0%), tests_pri_0: 655 (11.8%), check_dkim_signature: 0.47 (0.0%), check_dkim_adsp: 3.5 (0.1%), tests_pri_500: 4841 (87.3%), poll_dns_idle: 4835 (87.2%), rewrite_mail: 0.00 (0.0%) Subject: Re: [PATCH 0/2] fix the traced mt-exec deadlock X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ebiederm@xmission.com (Eric W. Biederman) writes: > The big lesson for me, and what was not obvious from your change > description is that we are changing the user space visible semantics > of exec+ptrace and that cred_guard_mutex is not at all the problem (as > we always take cred_guard_mutex in a killable or interruptible way). Just to follow up. Because the cred_guard_mutex is fine as is we don't need to move de_thread out from under cred_guard_mutex. We just need to change de_thread to wait until all of the other threads are zombies. Which should remove about half your proposed patch. The other key thing is that knowning it isn't cred_guard_mutex let's us know that this kind of deadlock goes all of the way back to when CLONE_THREAD was merged into the kernel. Insteresingly enough looking at zap_other_threads and notify_count I have found a second bug. When a multi-threaded processes becomes a zombie we don't send the notification to the parent process until the non-leader threads have been reaped. Which means ptrace can mess up sending SIGCHLD to the parent. Now arguably that might be what is desirable but I don't think so. If we aren't ptracing a thread then I don't think we want to delay sending SIGCHLD to the parent. So this whole area of the semantics of a ptrace'd multi-threaded process exiting/exec'ing looks like it needs a thorough going over. Eric