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

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Date:	Thu, 20 Nov 2008 10:10:08 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.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

Oleg Nesterov <oleg@...hat.com> writes:

> 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.

The signal handlers should still be the same.

>> 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 ;)

Could be.

> 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.

Interesting point.

>> 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.

Could be.  I have to follow up on what craziness upstart is doing.
So my information is a bit dated.

> 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.

Like I said I need to retest.  I was on a 2.6.26 fedora kernel base.
So if there have been recent bug fixes things may have changed.

Eric
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