linux-kernel - Re: SELinux lead to soft lockup when pid 1 proceess reap child

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Date:   Mon, 9 Jan 2017 16:26:02 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>,
        Oleg Nesterov <oleg@...hat.com>,
        yangshukui <yangshukui@...wei.com>
Cc:     selinux@...ho.nsa.gov, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Guohanjun (Hanjun Guo)" <guohanjun@...wei.com>,
        'Qiang Huang' <h.huangqiang@...wei.com>,
        Lizefan <lizefan@...wei.com>, "miaoxie (A)" <miaoxie@...wei.com>,
        Zhangdianfang <zhangdianfang@...wei.com>, paul@...l-moore.com,
        eparis@...isplace.org, james.l.morris@...cle.com,
        ebiederm@...ssion.com, serge.hallyn@...ntu.com
Subject: Re: SELinux lead to soft lockup when pid 1 proceess reap child

On 1/9/2017 10:43 AM, Stephen Smalley wrote:
> On Mon, 2017-01-09 at 19:29 +0100, Oleg Nesterov wrote:
>> Seriously, could someone explain why do we need the
>> security_task_wait()
>> hook at all?
> I would be ok with killing it.
> IIRC, the original motivation was to block an unauthorized data flow
> from child to parent when the child context differs, but part of that
> original design was also to reparent the child automatically, and that
> was never implemented.  I don't think there is a real use case for it
> in practice and it just breaks things, so let's get rid of it unless
> someone objects.

A strict Bell & LaPadula sensitivity model must prohibit a child
with a more sensitive label from signalling its parent. Except that
Bad Things happen when you try enforcing that on a real system.
I agree with Stephen and Oleg that this hook could go away and not
be missed. If someone *really* wants to implement a strict B&L
policy I believe that a reparentting solution is going to be necessary
anyway.

Regardless of the outcome, I notice that the Smack hook does not
do anything, and that's unnecessary overhead, so it's going to come
out.

>
>>
>> On 01/09, Oleg Nesterov wrote:
>>>
>>> On 01/09, yangshukui wrote:
>>>>
>>>> --- a/security/selinux/hooks.c
>>>> +++ b/security/selinux/hooks.c
>>>> @@ -3596,6 +3596,9 @@ static int selinux_task_kill(struct
>>>> task_struct *p,
>>>> struct siginfo *info,
>>>>
>>>>  static int selinux_task_wait(struct task_struct *p)
>>>>  {
>>>> +       if (pid_vnr(task_tgid(current)) == 1){
>>>> +                return 0;
>>> this check is not really correct, it can be a sub-thread... Doesn't
>>> matter,
>>> please see below.
>>>
>>>> +       }
>>>>         return task_has_perm(p, current, PROCESS__SIGCHLD);
>>>>  }
>>>> It work but it permit pid 1 process to reap child without selinux
>>>> check. Can
>>>> we have a better way to handle this problem?
>>> I never understood why security_task_wait() should deny to reap a
>>> child. But
>>> since it can we probably want some explicit "the whole namespace
>>> goes away" check.
>>> We could use, say, PIDNS_HASH_ADDING but I'd suggest something like
>>> a trivial change
>>> below for now.
>>>
>>> Eric, what do you think?
>>>
>>> Oleg.
>>>
>>> diff --git a/security/security.c b/security/security.c
>>> index f825304..1330b4e 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -1027,6 +1027,9 @@ int security_task_kill(struct task_struct *p,
>>> struct siginfo *info,
>>>  
>>>  int security_task_wait(struct task_struct *p)
>>>  {
>>> +	/* must be the exiting child reaper */
>>> +	if (unlikely(current->flags & PF_EXITING))
>>> +		return 0;
>>>  	return call_int_hook(task_wait, 0, p);
>>>  }
>>>  
> --
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