Re: [PATCH] kthread: fix use-after-free if kthread fork fails

[Vegard Nossum]

On 05/05/17 18:44, Oleg Nesterov wrote:
> On 05/05, Vegard Nossum wrote:
> > If a kthread forks (e.g. usermodehelper since commit 1da5c46fa965) but
> > fails in copy_process() between calling dup_task_struct() and setting
> > p->set_child_tid, then the value of p->set_child_tid will be inherited
> > from the parent and get prematurely freed by free_kthread_struct().
>
> Aaah... thanks!
>
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -518,6 +518,13 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
> >  	atomic_set(&tsk->stack_refcount, 1);
> >  #endif
> >
> > +	/*
> > +	 * Forking kthreads (e.g. usermodehelper) should not inherit this
> > +	 * field since it's a pointer to a 'struct kthread' which is not
> > +	 * reference counted.
> > +	 */
> > +	tsk->set_child_tid = NULL;
> > +
>
> Can't we just move both
>
> 	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
> 	/*
> 	 * Clear TID on mm_release()?
> 	 */
> 	p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
>
> lines here?

clone_flags is not available in dup_task_struct(), but we could move
those lines higher in copy_process(). The reason we didn't do it was
that we thought it was a little fragile/unobvious that this has to
happen before free_task() is called and that it was safer to clear it in
dup_task_struct() (which also contains zeroing of other fields).

The newly attached patch has been tested and seems to work, if you
prefer it.


Vegard
diff --git a/kernel/fork.c b/kernel/fork.c
index fbdc29365b83..c52e22fdf7ca 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1561,6 +1561,18 @@ static __latent_entropy struct task_struct *copy_process(
 	if (!p)
 		goto fork_out;
 
+	/*
+	 * This _must_ happen before we call free_task(), i.e. before we jump
+	 * to any of the bad_fork_* labels. This is to avoid freeing
+	 * p->set_child_tid which is (ab)used as a kthread's data pointer for
+	 * kernel threads (PF_KTHREAD).
+	 */
+	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
+	/*
+	 * Clear TID on mm_release()?
+	 */
+	p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
+
 	ftrace_graph_init_task(p);
 
 	rt_mutex_init_task(p);
@@ -1727,11 +1739,6 @@ static __latent_entropy struct task_struct *copy_process(
 		}
 	}
 
-	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
-	/*
-	 * Clear TID on mm_release()?
-	 */
-	p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;
 #ifdef CONFIG_BLOCK
 	p->plug = NULL;
 #endif