linux-kernel - Re: [PATCH v4] seccomp: add ptrace options for suspend/resume

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Date:	Wed, 10 Jun 2015 10:20:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tycho Andersen <tycho.andersen@...onical.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	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 v4] seccomp: add ptrace options for suspend/resume

On Wed, Jun 10, 2015 at 9:31 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/09, Andy Lutomirski wrote:
>>
>> On Tue, Jun 9, 2015 at 5:49 PM, Tycho Andersen
>> >
>> > @@ -556,6 +556,15 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
>> >         if (data & ~(unsigned long)PTRACE_O_MASK)
>> >                 return -EINVAL;
>> >
>> > +       if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
>
> Well, we should do this if
>
>                         (data & O_SUSPEND) && !(flags & O_SUSPEND)
>
> or at least if
>
>                         (data ^ flags) & O_SUSPEND
>
>
>> > +               if (!config_enabled(CONFIG_CHECKPOINT_RESTORE) ||
>> > +                   !config_enabled(CONFIG_SECCOMP))
>> > +                       return -EINVAL;
>> > +
>> > +               if (!capable(CAP_SYS_ADMIN))
>> > +                       return -EPERM;
>>
>> I tend to think that we should also require that current not be using
>> seccomp.  Otherwise, in principle, there's a seccomp bypass for
>> privileged-but-seccomped programs.
>
> Andy, I simply can't understand why do we need any security check at all.
>
> OK, yes, in theory we can have a seccomped CAP_SYS_ADMIN process, seccomp
> doesn't filter ptrace, you hack that process and force it to attach to
> another CAP_SYS_ADMIN/seccomped process, etc, etc... Looks too paranoid
> to me.

I've sometimes considered having privileged processes I write fork and
seccomp their child.  Of course, if you're allowing ptrace through
your seccomp filter, you open a giant can of worms, but I think we
should take the more paranoid approach to start and relax it later as
needed.  After all, for the intended use of this patch, stuff will
break regardless of what we do if the ptracer is itself seccomped.

I could be convinced that if the ptracer is outside seccomp then we
shouldn't need the CAP_SYS_ADMIN check.  That would at least make this
work in a user namespace.

>> > @@ -590,6 +590,10 @@ void secure_computing_strict(int this_syscall)
>> >  {
>> >         int mode = current->seccomp.mode;
>> >
>> > +       if (config_enabled(CONFIG_CHECKPOINT_RESTORE) &&
>> > +           unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
>> > +               return;
>> > +
>> >         if (mode == 0)
>> >                 return;
>> >         else if (mode == SECCOMP_MODE_STRICT)
>> > @@ -691,6 +695,10 @@ u32 seccomp_phase1(struct seccomp_data *sd)
>> >         int this_syscall = sd ? sd->nr :
>> >                 syscall_get_nr(current, task_pt_regs(current));
>> >
>> > +       if (config_enabled(CONFIG_CHECKPOINT_RESTORE) &&
>> > +           unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
>> > +               return SECCOMP_PHASE1_OK;
>> > +
>>
>> If it's not hard, it might still be nice to try to fold this into
>> mode.  This code is rather hot.  If it would be a mess, then don't
>> worry about it for now.
>
> IMO, this would be a mess ;) At least compared to this simple patch.
>
> Suppose we add SECCOMP_MODE_SUSPENDED. Not only this adds the problems
> with detach if the tracer dies.
>
> We need to change copy_seccomp(). And it is not clear what should we
> do if the child is traced too.
>
> We need to change prctl_set_seccomp() paths.
>
> And even the "tracee->seccomp.mode = SECCOMP_MODE_SUSPENDED" code needs
> some locking even if the tracee is stopped, we need to avoid the races
> with SECCOMP_FILTER_FLAG_TSYNC from other threads.
>

Agreed.  Let's hold off until this becomes a problem (if it ever does).

--Andy
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