Re: [PATCH v2] seccomp: add ptrace options for suspend/resume

[Tycho Andersen <tycho.andersen@canonical.com>]

On Tue, Jun 09, 2015 at 03:06:07PM -0700, Kees Cook wrote:
> On Tue, Jun 9, 2015 at 2:52 PM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> > On Tue, Jun 09, 2015 at 02:45:49PM -0700, Kees Cook wrote:
> >> On Tue, Jun 9, 2015 at 2:22 PM, Tycho Andersen
> >> <tycho.andersen@canonical.com> wrote:
> >> > Hi Kees, Andy,
> >> >
> >> > On Fri, Jun 05, 2015 at 11:16:50PM +0200, Oleg Nesterov wrote:
> >> >> Hi Tycho,
> >> >>
> >> >> On 06/04, Tycho Andersen wrote:
> >> >> > > > +#ifdef CONFIG_CHECKPOINT_RESTORE
> >> >> > > > +bool may_suspend_seccomp(void)
> >> >> > > > +{
> >> >> > > > +       if (!capable(CAP_SYS_ADMIN))
> >> >> > > > +               return false;
> >> >> > > > +
> >> >> > > > +       if (current->seccomp.mode != SECCOMP_MODE_DISABLED)
> >> >> > > > +               return false;
> >> >> > >
> >> >> > > Heh. OK, I won't argue with the new check too ;)
> >> >> >
> >> >> > Actually now that I think about it I agree with you, these checks
> >> >> > don't seem necessary. Even inside a user namespace, if you can ptrace
> >> >> > a process you can make it do whatever you want irrespective of
> >> >> > seccomp, as long as it has the necessary capabilities. Once the
> >> >> > seccomp checks are run after ptrace, they'll be enforced so you
> >> >> > couldn't have it call whatever you want in the first place.
> >> >>
> >> >> Good ;)
> >> >>
> >> >> > Still, perhaps I'm missing something...
> >> >>
> >> >> Kees, Andy?
> >> >
> >> > Any thoughts on removing may_suspend_seccomp() all together?
> >>
> >> As in, just open-code the check? That would be fine by me.
> >
> > Sorry, I meant getting rid of any checks entirely. Using my argument
> > above I've managed to convince myself they don't add any value. You
> > guys know a lot more about this than I do, though.
> 
> Well, as things stand currently, yes, that check would be redundant.
> The fact that ptrace can be used to bypass seccomp is kind of an
> accident, though. The design for ptrace-based seccomp managers was
> that the manager would do the work, rather than rewriting the syscall
> on behalf of the child. I don't think anything actually uses this
> effect. It's something we've wanted to fix, though a clean solution
> isn't obvious. As a result, I'm cautious to add this behavior in such
> a wide open fashion. For now, I'd like to limit the scope of this to
> CAP_SYS_ADMIN.
> 
> I do think dropping the seccomp.mode check is fine -- this would mean
> you could set this flag before the child even added seccomp filters.
> So, instead of the function call, maybe just add the capable() call?

Ok, sounds good; I'll make the change and re-send.

Thanks!

Tycho