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

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Date:	Tue, 2 Jun 2015 20:28:29 +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] seccomp: add ptrace commands for suspend/resume

On 06/01, Tycho Andersen wrote:
>
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -25,6 +25,9 @@ struct seccomp_filter;
>  struct seccomp {
>  	int mode;
>  	struct seccomp_filter *filter;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	bool suspended;
> +#endif

Then afaics you need to change copy_seccomp() to clear ->suspended.
At least if the child is not traced.

> @@ -691,6 +697,11 @@ u32 seccomp_phase1(struct seccomp_data *sd)
>  	int this_syscall = sd ? sd->nr :
>  		syscall_get_nr(current, task_pt_regs(current));
>  
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	if (unlikely(current->seccomp.suspended))
> +		return SECCOMP_PHASE1_OK;
> +#endif
> +

I am wondering if PTRACE_SUSPEND_SECCOMP can just clear/set TIF_SECCOMP.
Of course, it is not that resume_seccomp() can simply do set_tsk_thread_flag,
it should be more careful. And prctl_set_seccomp() paths will need some
changes. Probably not, this would be more complex.

So perhaps it would be better to add PTRACE_O_SUSPEND_SECCOMP? This also
solves the problem with the killed tracer. Except TIF_NOTSC...

But why do we bother to play with TIF_NOTSC, could you explain?

> +int suspend_seccomp(struct task_struct *task)
> +{
> +	int ret = -EACCES;
> +
> +	spin_lock_irq(&task->sighand->siglock);
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		goto out;

I am puzzled ;) Why do we need ->siglock? And even if we need it, why
we can't check CAP_SYS_ADMIN lockless?

And I am not sure I understand why do we need the additional security
check, but I leave this to you and Andy.

If you have the rights to trace this task, then you can do anything
the tracee could do without the filtering.

> +
> +	task->seccomp.suspended = true;
> +
> +#ifdef TIF_NOTSC
> +	if (task->seccomp.mode == SECCOMP_MODE_STRICT)
> +		clear_tsk_thread_flag(task, TIF_NOTSC);
> +#endif
> +
> +	ret = 0;
> +out:
> +	spin_unlock_irq(&task->sighand->siglock);
> +
> +	return ret;
> +}
> +
> +int resume_seccomp(struct task_struct *task)
> +{
> +	int ret = -EACCES;
> +
> +	spin_lock_irq(&task->sighand->siglock);
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		goto out;
> +
> +	task->seccomp.suspended = false;
> +
> +#ifdef TIF_NOTSC
> +	if (task->seccomp.mode == SECCOMP_MODE_STRICT)
> +		set_tsk_thread_flag(task, TIF_NOTSC);
> +#endif
> +
> +	ret = 0;
> +out:
> +	spin_unlock_irq(&task->sighand->siglock);
> +
> +	return ret;
> +}
> +#endif /* CONFIG_CHECKPOINT_RESTORE */

Well, I do not think we need 2 helpers, just one which takes a boolean
will look better, imo.

Oleg.

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