Re: [PATCH review 1/3] pidns: Outlaw thread creation after unshare(CLONE_NEWPID)

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Rob Landley <rob-VoJi6FS/r0vR7s880joybQ@public.gmane.org> writes:

> On 12/21/2012 10:57:34 PM, Eric W. Biederman wrote:
>> 
>> The sequence:
>> unshare(CLONE_NEWPID)
>> clone(CLONE_THREAD|CLONE_SIGHAND|CLONE_VM)
>> 
>> Creates a new process in the new pid namespace without setting
>> pid_ns->child_reaper.  After forking this results in a NULL
>> pointer dereference.
>> 
>> Avoid this and other nonsense scenarios that can show up after
>> creating a new pid namespace with unshare by adding a new
>> check in copy_prodcess.
>> 
>> Pointed-out-by:  Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>> Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
>> ---
>>  kernel/fork.c |    8 ++++++++
>>  1 files changed, 8 insertions(+), 0 deletions(-)
>> 
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index a31b823..65ca6d2 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1166,6 +1166,14 @@ static struct task_struct  
>> *copy_process(unsigned long clone_flags,
>>  				current->signal->flags &  
>> SIGNAL_UNKILLABLE)
>>  		return ERR_PTR(-EINVAL);
>> 
>> +	/*
>> +	 * If the new process will be in a different pid namespace
>> +	 * don't allow the creation of threads.
>> +	 */
>> +	if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
>> +	    (task_active_pid_ns(current) != current->nsproxy->pid_ns))
>> +		return ERR_PTR(-EINVAL);
>> +
>
> Since the first bit will trigger if clone_flags has just CLONE_VM  
> without CLONE_NEWPID, or vice versa, I'm guessing this is a fast path  
> optimization? (Otherwise you meant (clone_flags &  
> (CLONE_VM|CLONE_NEWPID)) == CLONE_VM|CLONE_NEWPID ?)
>
> (Just trying to wrap my head around it...)

Actually I mean (clone_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM | CLONE_NEWPID))
CLONE_THREAD and CLONE_SIGHAND imply CLONE_VM... and that is enfored above.

I don't mean all of those flags must be in place.

CLONE_NEWPID is an optimization in that the test is also in copy_pid_ns
but there is no point in going to all of the work to get there if we are
going to be testing for this scenario anyway.

I definitely don't mean (clone_flags & (CLONE_VM|CLONE_NEWPID)) ==
(CLONE_VM|CLONE_NEWPID)).  The task_active_pid_ns(current) !=
current->nsproxy->pid_ns case is what tests to see if the pid namespace
has already been unshared.

The sequence "unshare(CLONE_NEWPID); unshare(CLONE_NEWPID);" is nonsense
and that is what CLONE_NEWPID is about in that test.

Similary the sequence "unshare(CLONE_NEWPID); clone(CLONE_THREAD);" is
nonsense and what the CLONE_VM is the test is for.  There are also a
number of other nonsense thread like states that CLONE_VM also catches
and prevents.

Eric