On 05/31, Michal Hocko wrote:
>
> On Mon 30-05-16 21:28:57, Oleg Nesterov wrote:
> >
> > I don't think we can trust vfork_done != NULL.
> >
> > copy_process() doesn't disallow CLONE_VFORK without CLONE_VM, so with this patch
> > it would be trivial to make the exploit which hides a memory hog from oom-killer.
>
> OK, I wasn't aware of this possibility.
Neither was me ;) I noticed this during this review.
> > Or I am totally confused?
>
> I cannot judge I am afraid. You are definitely much more familiar with
> all these subtle details than me.
OK, I just verified that clone(CLONE_VFORK|SIGCHLD) really works to be sure.
> +/* expects to be called with task_lock held */
> +static inline bool in_vfork(struct task_struct *tsk)
> +{
> + bool ret;
> +
> + /*
> + * need RCU to access ->real_parent if CLONE_VM was used along with
> + * CLONE_PARENT
> + */
> + rcu_read_lock();
> + ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
> + rcu_read_unlock();
> +
> + return ret;
> +}
Yes, but may I ask to add a comment? And note that "expects to be called with
task_lock held" looks misleading, we do not need the "stable" tsk->vfork_done
since we only need to check if it is NULL or not.
It would be nice to explain that
1. we check real_parent->mm == tsk->mm because CLONE_VFORK does not
imply CLONE_VM
2. CLONE_VFORK can be used with CLONE_PARENT/CLONE_THREAD and thus
->real_parent is not necessarily the task doing vfork(), so in
theory we can't rely on task_lock() if we want to dereference it.
And in this case we can't trust the real_parent->mm == tsk->mm
check, it can be false negative. But we do not care, if init or
another oom-unkillable task does this it should blame itself.
Oleg.
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