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
