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

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

[Oleg Nesterov]

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);
 }
 

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

[Oleg Nesterov]

Seriously, could someone explain why do we need the security_task_wait()
hook at all?


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);
>  }
>  

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

[Stephen Smalley]

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.

> 
> 
> 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);
> >  }
> >  

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

[Paul Moore]

On Mon, Jan 9, 2017 at 1:43 PM, Stephen Smalley <sds@tycho.nsa.gov> 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.

Patches are always welcome, plenty of time to get things in for 4.11 :)

-- 
paul moore
www.paul-moore.com

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

[Casey Schaufler]

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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