From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-f193.google.com ([209.85.167.193]:34066 "EHLO mail-oi1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726496AbeJIUxa (ORCPT ); Tue, 9 Oct 2018 16:53:30 -0400 Received: by mail-oi1-f193.google.com with SMTP id v69-v6so1233914oif.1 for ; Tue, 09 Oct 2018 06:36:30 -0700 (PDT) MIME-Version: 1.0 References: <20180927151119.9989-1-tycho@tycho.ws> <20180927151119.9989-4-tycho@tycho.ws> <20181008151629.hkgzzsluevwtuclw@brauner.io> <20181008162147.ubfxxsv2425l2zsp@brauner.io> <20181008181815.pwnqxngj22mhm2vj@brauner.io> <20181009132850.fp6yne2vgmfpi27k@brauner.io> In-Reply-To: <20181009132850.fp6yne2vgmfpi27k@brauner.io> From: Jann Horn Date: Tue, 9 Oct 2018 15:36:04 +0200 Message-ID: Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace To: christian@brauner.io 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: +cc selinux people explicitly, since they probably have opinions on this On Tue, Oct 9, 2018 at 3:29 PM Christian Brauner wrote: > On Tue, Oct 09, 2018 at 02:39:53PM +0200, Jann Horn wrote: > > On Mon, Oct 8, 2018 at 8:18 PM Christian Brauner wrote: > > > On Mon, Oct 08, 2018 at 06:42:00PM +0200, Jann Horn wrote: > > > > On Mon, Oct 8, 2018 at 6:21 PM Christian Brauner wrote: > > > > > On Mon, Oct 08, 2018 at 05:33:22PM +0200, Jann Horn wrote: > > > > > > On Mon, Oct 8, 2018 at 5:16 PM Christian Brauner wrote: > > > > > > > On Thu, Sep 27, 2018 at 09:11:16AM -0600, Tycho Andersen wrote: > > > > > > > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c > > > > > > > > index 44a31ac8373a..17685803a2af 100644 > > > > > > > > --- a/kernel/seccomp.c > > > > > > > > +++ b/kernel/seccomp.c > > > > > > > > @@ -1777,4 +1777,35 @@ static struct file *init_listener(struct task_struct *task, > > > > > > > > > > > > > > > > return ret; > > > > > > > > } > > > > > > > > + > > > > > > > > +long seccomp_new_listener(struct task_struct *task, > > > > > > > > + unsigned long filter_off) > > > > > > > > +{ > > > > > > > > + struct seccomp_filter *filter; > > > > > > > > + struct file *listener; > > > > > > > > + int fd; > > > > > > > > + > > > > > > > > + if (!capable(CAP_SYS_ADMIN)) > > > > > > > > + return -EACCES; > > > > > > > > > > > > > > I know this might have been discussed a while back but why exactly do we > > > > > > > require CAP_SYS_ADMIN in init_userns and not in the target userns? What > > > > > > > if I want to do a setns()fd, CLONE_NEWUSER) to the target process and > > > > > > > use ptrace from in there? > > > > > > > > > > > > See https://lore.kernel.org/lkml/CAG48ez3R+ZJ1vwGkDfGzKX2mz6f=jjJWsO5pCvnH68P+RKO8Ow@mail.gmail.com/ > > > > > > . Basically, the problem is that this doesn't just give you capability > > > > > > over the target task, but also over every other task that has the same > > > > > > filter installed; you need some sort of "is the caller capable over > > > > > > the filter and anyone who uses it" check. > > > > > > > > > > Thanks. > > > > > But then this new ptrace feature as it stands is imho currently broken. > > > > > If you can install a seccomp filter with SECCOMP_RET_USER_NOTIF if you > > > > > are ns_cpabable(CAP_SYS_ADMIN) and also get an fd via seccomp() itself > > > > > if you are ns_cpabable(CAP_SYS_ADMIN) > > > > Actually, you don't need CAP_SYS_ADMIN for seccomp() at all as long as > > you enable the NNP flag, I think? > > Yes, if you turn on NNP you don't even need sys_admin. > > > > > > > > then either the new ptrace() api > > > > > extension should be fixed to allow for this too or the seccomp() way of > > > > > retrieving the pid - which I really think we want - needs to be fixed to > > > > > require capable(CAP_SYS_ADMIN) too. > > > > > The solution where both require ns_capable(CAP_SYS_ADMIN) is - imho - > > > > > the preferred way to solve this. > > > > > Everything else will just be confusing. > > > > > > > > First you say "broken", then you say "confusing". Which one do you mean? > > > > > > Both. It's broken in so far as it places a seemingly unnecessary > > > restriction that could be fixed. You outlined one possible fix yourself > > > in the link you provided. > > > > If by "possible fix" you mean "check whether the seccomp filter is > > only attached to a single task": That wouldn't fundamentally change > > the situation, it would only add an additional special case. > > > > > And it's confusing in so far as there is a way > > > via seccomp() to get the fd without said requirement. > > > > I don't find it confusing at all. seccomp() and ptrace() are very > > Fine, then that's a matter of opinion. I find it counterintuitive that > you can get an fd without privileges via one interface but not via > another. > > > different situations: When you use seccomp(), infrastructure is > > Sure. Note, that this is _one_ of the reasons why I want to make sure we > keep the native seccomp() only based way of getting an fd without > forcing userspace to switching to a differnet kernel api. > > > already in place for ensuring that your filter is only applied to > > processes over which you are capable, and propagation is limited by > > inheritance from your task down. When you use ptrace(), you need a > > pretty different sort of access check that checks whether you're > > privileged over ancestors, siblings and so on of the target task. > > So, don't get me wrong I'm not arguing against the ptrace() interface in > general. If this is something that people find useful, fine. But, I > would like to have a simple single-syscall pure-seccomp() based way of > getting an fd, i.e. what we have in patch 1 of this series. Yeah, I also prefer the seccomp() one. > > But thinking about it more, I think that CAP_SYS_ADMIN over the saved > > current->mm->user_ns of the task that installed the filter (stored as > > a "struct user_namespace *" in the filter) should be acceptable. > > Hm... Why not CAP_SYS_PTRACE? Because LSMs like SELinux add extra checks that apply even if you have CAP_SYS_PTRACE, and this would subvert those. The only capability I know of that lets you bypass LSM checks by design (if no LSM blocks the capability itself) is CAP_SYS_ADMIN. > 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". > > 7. because the seccomp filter is shared by task A and task B, task C > > is now able to influence syscall results for syscalls performed by > > task A > > [1]: https://lore.kernel.org/lkml/CAG48ez3R+ZJ1vwGkDfGzKX2mz6f=jjJWsO5pCvnH68P+RKO8Ow@mail.gmail.com/