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

Andreea Bernat
2014-08-29T17:57:31Z

On Thu, Aug 28, 2014 at 12:32:31PM +0100, David Howells wrote:
> 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;

Looks good to me.

Regards,
Andreea

>