Re: [PATCH 1/2] device_cgroup: fix RCU usage

[Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>]

Quoting Tejun Heo (tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org):
> dev_cgroup->exceptions is protected with devcgroup_mutex for writes
> and RCU for reads; however, RCU usage isn't correct.
> 
> * dev_exception_clean() doesn't use RCU variant of list_del() and
>   kfree().  The function can race with may_access() and may_access()
>   may end up dereferencing already freed memory.  Use list_del_rcu()
>   and kfree_rcu() instead.
> 
> * may_access() may be called only with RCU read locked but doesn't use
>   RCU safe traversal over ->exceptions.  Use list_for_each_entry_rcu().
> 
> Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Cc: Aristeu Rozanski <aris-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> Cc: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>

Acked-by: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>

thanks,
-serge

> ---
> Oops, wrong patch.  This is the correct one.
> 
> Thanks.
> 
>  security/device_cgroup.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -164,8 +164,8 @@ static void dev_exception_clean(struct d
>  	struct dev_exception_item *ex, *tmp;
>  
>  	list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
> -		list_del(&ex->list);
> -		kfree(ex);
> +		list_del_rcu(&ex->list);
> +		kfree_rcu(ex, rcu);
>  	}
>  }
>  
> @@ -298,7 +298,7 @@ static int may_access(struct dev_cgroup
>  	struct dev_exception_item *ex;
>  	bool match = false;
>  
> -	list_for_each_entry(ex, &dev_cgroup->exceptions, list) {
> +	list_for_each_entry_rcu(ex, &dev_cgroup->exceptions, list) {
>  		if ((refex->type & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
>  			continue;
>  		if ((refex->type & DEV_CHAR) && !(ex->type & DEV_CHAR))