Re: [PATCH RFC] rcu: assoc_array: Add critical section to avoid a bug

David Howells
2014-08-28T11:32:43Z

Andreea-Cristina Bernat <bernat.ada@gmail.com> wrote:

> * The function "assoc_array_gc()" could be preempted between the call to
> "assoc_array_apply_edit()" call and the assignment
> "edit->array->nr_leaves_on_tree = nr_leaves_on_tree;", so the grace
> period could complete.

The bug is real, but this patch isn't the right solution.

The edit script should be considered inaccessible to a function the moment it
calls assoc_array_apply_edit() or assoc_array_cancel_edit(). I think this is
a better way.

David
---
assoc_array.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

commit 10c26fcc224c0515e15272515e7b9006cb08adc8
Author: David Howells <dhowells@redhat.com>
Date: Wed Aug 27 18:39:44 2014 +0100

KEYS: Fix use-after-free in assoc_array_gc()

An edit script should be considered inaccessible by a function once it has
called assoc_array_apply_edit() or assoc_array_cancel_edit().

However, assoc_array_gc() is accessing the edit script just after the
gc_complete: label.

Reported-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
index c0b1007011e1..ae146f0734eb 100644
--- a/lib/assoc_array.c
+++ b/lib/assoc_array.c
@@ -1735,7 +1735,7 @@ ascend_old_tree:
gc_complete:
edit->set[0].to = new_root;
assoc_array_apply_edit(edit);
- edit->array->nr_leaves_on_tree = nr_leaves_on_tree;
+ array->nr_leaves_on_tree = nr_leaves_on_tree;
return 0;