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

[Christian Brauner <christian@brauner.io>]

On Tue, Oct 09, 2018 at 02:39:53PM +0200, Jann Horn wrote:
> On Mon, Oct 8, 2018 at 8:18 PM Christian Brauner <christian@brauner.io> 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 <christian@brauner.io> 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 <christian@brauner.io> 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.

> 
> 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?


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;

> 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/