linux-kernel - Re: pppd service crash in linux-3.13.6

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Message-ID: <20140314210456.GA19032@redhat.com>
Date:	Fri, 14 Mar 2014 22:04:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Sree Harsha Totakura <sreeharsha@...akura.in>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: pppd service crash in linux-3.13.6

On 03/14, Peter Hurley wrote:
>
> On 03/14/2014 03:23 PM, Oleg Nesterov wrote:
>> On 03/14, Peter Hurley wrote:
>>>
> Yes, cgroup_release_agent() is the work function that is scheduled.
>
>>> which requires both namespace and tty facilities.
>>
>> Hmm... why?
>>
>> The exiting task obviously can't exec. The only way to spawn a userspace
>> process is call_usermodehelper(), it should work just fine, no?
>
> You're correct, in the immediate sense that the user command exec'd will
> not inherit open file descriptors.
>
> But what if it expects to be able to find the intact children of
> the foreground process group, and can't because the controlling tty
> has already been torn down and all the children already sent SIGHUP.

Which group/tty ? call_usermodehelper() asks the workqueue thread
to kthread_create/exec. See also below...

> Or what if the user command expects to find and join the user namespace
> of the dying process but now it's already been freed?

But it can't even know who called call_usermodehelper(). Besides,
cgroup_release_agent() uses UMH_WAIT_EXEC, so the caller can continue
and disappear completely before the usermode process has any chance
to do something.

Oleg.

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