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=-2.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 56E7EC43441 for ; Wed, 10 Oct 2018 13:18:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F077F214DA for ; Wed, 10 Oct 2018 13:18:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=brauner.io header.i=@brauner.io header.b="bcjsOpho" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F077F214DA Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=brauner.io Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727007AbeJJUks (ORCPT ); Wed, 10 Oct 2018 16:40:48 -0400 Received: from mail-wr1-f66.google.com ([209.85.221.66]:34765 "EHLO mail-wr1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726206AbeJJUks (ORCPT ); Wed, 10 Oct 2018 16:40:48 -0400 Received: by mail-wr1-f66.google.com with SMTP id l6-v6so5299921wrt.1 for ; Wed, 10 Oct 2018 06:18:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=brauner.io; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to :user-agent; bh=mrNWOe4D332Ao3uTuRsJGgtjrAka4HbmLpkdyldlMQE=; b=bcjsOphoOHTGWG312QANtPLIwMDdataW2a1nU958uGzG+sw9gVD/1rnxNPVaZGkZ1b bmBgarh1SQ0SYDktmGXSbJ5VajrPqnqDQqXkTWy44yDKeo825w1xyR0HJ7V1jOuU6QOc 55d/H0/AVsf6Tto3kaWyhb+n6oPJLX3QNPizN8iGJP+mwlz/0hjFk0WtXvd9f+goG9UY +SLPDYst/src4XOKDMhXLH1f5YpTbZHOAbYpWzMlQqNVfDRzUwOLjFkoBm3zXh63Okuc O2txvrx4Noih4/qF6f+3kV6D/Y6KXDAp38uCEoQn7wB36//odpAO1aQ1Vfh+yQe2FCiN d3Pg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to:user-agent; bh=mrNWOe4D332Ao3uTuRsJGgtjrAka4HbmLpkdyldlMQE=; b=pvzeKTm9fOGgUr4B+X/2+h+PRJC8x8nRLQVPPGQW7iAWhiLQOz5/t9QfwdKilj3R4E 5+7XNj2zOpS8IsV7lxk24xU13l5EiYoFlWWTSp7CBb8P4OaeNpg76LESLan87oOWJ0Gz ihBX+OeQHqIx+XWuttJxuRJ2Xownp5M2V6NriKRQXU0d3s+9VtZj+7dNSsoJY+UoWCg8 oDLlGUofKUHpsDeDS26y2Q9oyweqAPcxHMYgbZsw2R20fXFZIOj+qMFWC24VBl6K21QV rrpUz7jUphq7vfvSb4STkDqjS4ZbCp7a1Jo9QFs8hTF7mVx00/xkGxkF+2O9qQxxysNI 58mg== X-Gm-Message-State: ABuFfohHudlGomoCnMNNXbYN0M0kz+pN7/D1F0PCM18edP5gHOZwellk DAM0VuVE26d1UndEkOTcdypzptRnkDACnw== X-Google-Smtp-Source: ACcGV63H48agUJ1un3WFx3Ad2N8e7J3MCPDHE1TkSVigtVvaG6M2m/wqJDznLfHedn+zFP81QqbiOA== X-Received: by 2002:a5d:47cb:: with SMTP id l11-v6mr23627970wrs.195.1539177517344; Wed, 10 Oct 2018 06:18:37 -0700 (PDT) Received: from brauner.io ([2a02:8070:8895:9700:8197:8849:535a:4f00]) by smtp.gmail.com with ESMTPSA id a18sm17792956wrx.55.2018.10.10.06.18.35 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 10 Oct 2018 06:18:36 -0700 (PDT) Date: Wed, 10 Oct 2018 15:18:30 +0200 From: Christian Brauner To: Jann Horn Cc: Tycho Andersen , Kees Cook , Linux API , containers@lists.linux-foundation.org, suda.akihiro@lab.ntt.co.jp, Oleg Nesterov , kernel list , "Eric W. Biederman" , linux-fsdevel@vger.kernel.org, Christian Brauner , Andy Lutomirski , linux-security-module , selinux@tycho.nsa.gov, Paul Moore , Stephen Smalley , Eric Paris Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace Message-ID: <20181010131829.rlczow555y6gig5p@brauner.io> References: <20181009132850.fp6yne2vgmfpi27k@brauner.io> <20181009134923.2fvf5roghqgaj5gq@brauner.io> <20181009140932.e5w5lgbgucbl72kt@brauner.io> <20181009162022.d7fd2wibyq6xi6sg@brauner.io> <20181010125422.rouslofknxzvygzr@brauner.io> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 10, 2018 at 03:10:11PM +0200, Jann Horn wrote: > On Wed, Oct 10, 2018 at 2:54 PM Christian Brauner wrote: > > On Tue, Oct 09, 2018 at 06:26:47PM +0200, Jann Horn wrote: > > > On Tue, Oct 9, 2018 at 6:20 PM Christian Brauner wrote: > > > > On Tue, Oct 09, 2018 at 05:26:26PM +0200, Jann Horn wrote: > > > > > On Tue, Oct 9, 2018 at 4:09 PM Christian Brauner wrote: > > > > > > On Tue, Oct 09, 2018 at 03:50:53PM +0200, Jann Horn wrote: > > > > > > > On Tue, Oct 9, 2018 at 3:49 PM Christian Brauner wrote: > > > > > > > > On Tue, Oct 09, 2018 at 03:36:04PM +0200, Jann Horn wrote: > > > > > > > > > On Tue, Oct 9, 2018 at 3:29 PM Christian Brauner wrote: > > > > > > > > > > One more thing. Citing from [1] > > > > > > > > > > > > > > > > > > > > > I think there's a security problem here. Imagine the following scenario: > > > > > > > > > > > > > > > > > > > > > > 1. task A (uid==0) sets up a seccomp filter that uses SECCOMP_RET_USER_NOTIF > > > > > > > > > > > 2. task A forks off a child B > > > > > > > > > > > 3. task B uses setuid(1) to drop its privileges > > > > > > > > > > > 4. task B becomes dumpable again, either via prctl(PR_SET_DUMPABLE, 1) > > > > > > > > > > > or via execve() > > > > > > > > > > > 5. task C (the attacker, uid==1) attaches to task B via ptrace > > > > > > > > > > > 6. task C uses PTRACE_SECCOMP_NEW_LISTENER on task B > > > > > > > > > > > > > > > > > > > > Sorry, to be late to the party but would this really pass > > > > > > > > > > __ptrace_may_access() in ptrace_attach()? It doesn't seem obvious to me > > > > > > > > > > that it would... Doesn't look like it would get past: > > > > > > > > > > > > > > > > > > > > tcred = __task_cred(task); > > > > > > > > > > if (uid_eq(caller_uid, tcred->euid) && > > > > > > > > > > uid_eq(caller_uid, tcred->suid) && > > > > > > > > > > uid_eq(caller_uid, tcred->uid) && > > > > > > > > > > gid_eq(caller_gid, tcred->egid) && > > > > > > > > > > gid_eq(caller_gid, tcred->sgid) && > > > > > > > > > > gid_eq(caller_gid, tcred->gid)) > > > > > > > > > > goto ok; > > > > > > > > > > if (ptrace_has_cap(tcred->user_ns, mode)) > > > > > > > > > > goto ok; > > > > > > > > > > rcu_read_unlock(); > > > > > > > > > > return -EPERM; > > > > > > > > > > ok: > > > > > > > > > > rcu_read_unlock(); > > > > > > > > > > mm = task->mm; > > > > > > > > > > if (mm && > > > > > > > > > > ((get_dumpable(mm) != SUID_DUMP_USER) && > > > > > > > > > > !ptrace_has_cap(mm->user_ns, mode))) > > > > > > > > > > return -EPERM; > > > > > > > > > > > > > > > > > > Which specific check would prevent task C from attaching to task B? If > > > > > > > > > the UIDs match, the first "goto ok" executes; and you're dumpable, so > > > > > > > > > you don't trigger the second "return -EPERM". > > > > > > > > > > > > > > > > You'd also need CAP_SYS_PTRACE in the mm->user_ns which you shouldn't > > > > > > > > have if you did a setuid to an unpriv user. (But I always find that code > > > > > > > > confusing.) > > > > > > > > > > > > > > Only if the target hasn't gone through execve() since setuid(). > > > > > > > > > > > > Sorry if I want to know this in excessive detail but I'd like to > > > > > > understand this properly so bear with me :) > > > > > > - If task B has setuid()ed and prctl(PR_SET_DUMPABLE, 1)ed but not > > > > > > execve()ed then C won't pass ptrace_has_cap(mm->user_ns, mode). > > > > > > > > > > Yeah. > > > > > > > > > > > - If task B has setuid()ed, exeved()ed it will get its dumpable flag set > > > > > > to /proc/sys/fs/suid_dumpable > > > > > > > > > > Not if you changed all UIDs (e.g. by calling setuid() as root). In > > > > > that case, setup_new_exec() calls "set_dumpable(current->mm, > > > > > SUID_DUMP_USER)". > > > > > > > > Actually, looking at this when C is trying to PTRACE_ATTACH to B as an > > > > unprivileged user even if B execve()ed and it is dumpable C still > > > > wouldn't have CAP_SYS_PTRACE in the mm->user_ns unless it already is > > > > privileged over mm->user_ns which means it must be in an ancestor > > > > user_ns. > > > > > > Huh? Why would you need CAP_SYS_PTRACE for anything here? You can > > > ptrace another process running under your UID just fine, no matter > > > what the namespaces are. I'm not sure what you're saying. > > > > Sorry, I was out the door yesterday when answering this and was too > > brief. I forgot to mention: /proc/sys/kernel/yama/ptrace_scope. It > > should be enabled by default on nearly all distros > > "nearly all distros"? AFAIK it's off on Debian, for starters. And Yama > still doesn't help you if one of the tasks enters a new user namespace > or whatever. > > Yama is a little bit of extra, heuristic, **opt-in** hardening enabled > in some configurations. It is **not** a fundamental building block you > can rely on. > > > and even if not - > > which is an administrators choice - you can usually easily enable it via > > sysctl. > > Opt-in security isn't good enough. Kernel interfaces should still be > safe to use even on a system that has all the LSM stuff disabled in > the kernel config. Then ptrace() isn't, I guess? But see https://lists.linuxfoundation.org/pipermail/containers/2018-October/039567.html I don't care as long as we have a way of getting the fd without the CAP_SYS_ADMIN requirement throught seccomp(). > > > 1 ("restricted ptrace") [default value] > > When performing an operation that requires a PTRACE_MODE_ATTACH check, > > the calling process must either have the CAP_SYS_PTRACE capability in > > the user namespace of the target process or it must have a prede‐ fined > > relationship with the target process. By default, the predefined > > relationship is that the target process must be a descendant of the > > caller. > > > > If you don't have it set you're already susceptible to all kinds of > > other attacks > > Oh? Can you be more specific, please? I was referring to attacks where you attach to processes that run as your user but might expose in-memory credentials or other sensitive information, (essentially what the manpage is outlining). > > > and I'm still not convinced we need to bring out the big > > capable(CAP_SYS_ADMIN) gun here.