Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

Christian Brauner
2018-10-10T13:18:41Z

On Wed, Oct 10, 2018 at 03:10:11PM +0200, Jann Horn wrote:
> On Wed, Oct 10, 2018 at 2:54 PM Christian Brauner <christian@brauner.io> 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 <christian@brauner.io> 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 <christian@brauner.io> 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 <christian@brauner.io> 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 <christian@brauner.io> 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.