From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f66.google.com ([209.85.128.66]:40290 "EHLO mail-wm1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726206AbeJJUcM (ORCPT ); Wed, 10 Oct 2018 16:32:12 -0400 Received: by mail-wm1-f66.google.com with SMTP id z204-v6so5711182wmc.5 for ; Wed, 10 Oct 2018 06:10:02 -0700 (PDT) Date: Wed, 10 Oct 2018 15:09:54 +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: <20181010130953.buvyzvyrsarzjt7v@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: <20181010125422.rouslofknxzvygzr@brauner.io> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, Oct 10, 2018 at 02:54:22PM +0200, 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 and even if not - > which is an administrators choice - you can usually easily enable it via > sysctl. > > 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 and I'm still not convinced we need to bring out the big > capable(CAP_SYS_ADMIN) gun here. That being said, given that Tycho agreed to leave in the native seccomp() way of retrieving an fd from the task without the sys_admin restriction [1] which we prefer and if we merge it with aforementioned feature I care way less about whether or not the restriction is upheld for ptrace() or not. [1]: https://lists.linuxfoundation.org/pipermail/containers/2018-October/039553.html