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

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Date:	Fri, 5 Jun 2015 23:16:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tycho Andersen <tycho.andersen@...onical.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>,
	Roland McGrath <roland@...k.frob.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
Subject: Re: [PATCH v2] seccomp: add ptrace options for suspend/resume

Hi Tycho,

On 06/04, Tycho Andersen wrote:
>
> On Thu, Jun 04, 2015 at 08:31:49PM +0200, Oleg Nesterov wrote:
> > Also. Suppose that the tracer sets SUSPEND_SECCOMP and then drops
> > CAP_SYS_ADMIN. After that it can't set or clear other ptrace options.
>
> Is this a case we're concerned about? I think this should be ok (i.e.
> "don't do that" :).

Sure, I won't insist. Just this looks a bit confusing. I mean, if you
read this code it is not clear why may_suspend_seccomp() is called even
if the tracer changes other bits, and "data & PTRACE_O_SUSPEND" is true
only because the tracer does _not_ change this option.

IOW, imo the code will just look better if may_suspend_seccomp() is
called only when PTRACE_O_SUSPEND is set.  But this is minor, feel free
to ignore.

> > > +#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?

Oleg.

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