linux-kernel - Re: [PATCH 1/2] protect /sbin/init from unwanted signals more

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Date:	Thu, 20 Nov 2008 15:52:34 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] protect /sbin/init from unwanted signals more

On 11/19, Eric W. Biederman wrote:
>
> Roland McGrath <roland@...hat.com> writes:
>
> > With that, I wonder if the SIGNAL_UNKILLABLE checks in get_signal_to_deliver
> > and complete_signal are needed at all.  Hmm, I guess we do because this
> > doesn't affect blocked signals, so they might be unblocked and delivered.
> > (Note that since it doesn't affect blocked signals, this doesn't break init
> > using sigwait if it wanted to.)
>
> Ah.  That answers the question I had bouncing in the back of my head.

Even worse. The signal can be dequeued even before unblocked by the target.
complete_signal() can "redirect" this signal to another thread wich doesn't
block it.

> My original analysis of the situation was that we should not send blocked signals.
> Treating handler != SIG_DFL as a permission check.  Not as an optimization.
>
> Mostly because it is more consistent and uniform.
>
> inits today don't do anything with blocked signals.

(I guess you mean "with blocked SIG_DFL signals", otherwise this is
 too strong ;)

If init does exec and do not want to miss (say) SIGCHLD, the only option
is to block it before exec. And right after exec the handler is SIG_DFL.

> They explicitly ignore all signals,
> they don't want to deal with an enable those they do.

I do remember I had the (unrelated) bugreport which in particular showed
that user-space sends SIGUSR1 to init. Usually init has a handler and does
something in responce, but sometimes the handler is SIG_DFL. I don't
remember the distribution, ubuntu iirc.

Yes, this perhaps means init is not perfect, but still.

> Which reminds me.  I need to retest, but I had a case where I had a trivial init
> that set all signal handlers to SIG_IGN so it could ignore SIGCHLD.  And not
> all of it's children were getting reaped automagically.  Do we have a bug in
> the reparenting/reaping logic?

Ah... I thought this was already fixed... shouldn't reparent_thread()
check task_detached() after do_notify() ? like ptrace_exit() does.

Oleg.

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