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

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Date:	Wed, 3 Jun 2015 12:36:00 -0600
From:	Tycho Andersen <tycho.andersen@...onical.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	Andrey Wagin <avagin@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, 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] seccomp: add ptrace commands for suspend/resume

On Wed, Jun 03, 2015 at 06:54:51PM +0200, Oleg Nesterov wrote:
> On 06/03, Tycho Andersen wrote:
> >
> > On Tue, Jun 02, 2015 at 08:48:48PM +0200, Oleg Nesterov wrote:
> >
> > > Otherwise, if we use PTRACE_O_ instead, it goes away automatically if
> > > the tracer dies or does PTRACE_DETACH.
> >
> > IIRC the flag goes away, but we still have to do something in
> > __ptrace_unlink to clear the seccomp suspended, so I'm not sure if the
> > automatic-ness helps us.
> 
> But we do not need seccomp->suspended at all?
> 
> Unless I missed something PTRACE_O_ needs a one-liner patch (ignoring
> the defines in include files),
> 
> --- x/kernel/seccomp.c
> +++ x/kernel/seccomp.c
> @@ -692,6 +692,9 @@ u32 seccomp_phase1(struct seccomp_data *
>  	int this_syscall = sd ? sd->nr :
>  		syscall_get_nr(current, task_pt_regs(current));
>  
> +	if (unlikely(current->ptrace & PT_NAME_OF_THIS_OPTION))
> +		return OK;
> +
>  	switch (mode) {
>  	case SECCOMP_MODE_STRICT:
>  		__secure_computing_strict(this_syscall);  /* may call do_exit */
> 	
> 
> OK, and the same check in secure_computing_strict().
> 
> No?

One problem with this is that we still incur the runtime overhead of
checking, which I guess is a question of ptrace vs. seccomp
complexity.

Andy had suggested multiplexing seccomp->suspended into seccomp->mode
directly to avoid this, but the above still requires a check. We could
play with TIF_SECCOMP, but that has the same problems as playing with
TIF_NOTSC.

Thoughts?

Tycho
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