[PATCH] audit: fix use-after-free in audit_add_watch

[Richard Guy Briggs]

On 2018-06-18 15:27, Ronny Chevalier wrote:
> audit_add_watch stores locally krule->watch without taking a reference
> on watch. Then, it calls audit_add_to_parent, and uses the watch stored
> locally.
> 
> Unfortunately, it is possible that audit_add_to_parent updates
> krule->watch.
> When it happens, it also drops a reference of watch which
> could free the watch.
> 
> How to reproduce (with KASAN enabled):
> 
>     auditctl -w /etc/passwd -F success=0 -k test_passwd
>     auditctl -w /etc/passwd -F success=1 -k test_passwd2
> 
> The second call to auditctl triggers the use-after-free, because
> audit_to_parent updates krule->watch to use a previous existing watch
> and drops the reference to the newly created watch.
> 
> To fix the issue, we grab a reference of watch and we release it at the
> end of the function.

Nice catch.

This doesn't quite look right though.  What if audit_get_nd() fails?
How about we put that audit_get_watch(watch) right before
audit_find_parent()?

> Signed-off-by: Ronny Chevalier <ronny.chevalier at hp.com>
> ---
>  kernel/audit_watch.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
> index f1ba88994508..4bb8d7718fbc 100644
> --- a/kernel/audit_watch.c
> +++ b/kernel/audit_watch.c
> @@ -421,6 +421,14 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
>  
>  	mutex_unlock(&audit_filter_mutex);
>  
> +	/*
> +	 * When we will be calling audit_add_to_parent, krule->watch might have
> +	 * been updated and watch might have been freed.
> +	 * So we need to keep a reference of watch.
> +	 */
> +
> +	audit_get_watch(watch);
> +
>  	/* Avoid calling path_lookup under audit_filter_mutex. */
>  	ret = audit_get_nd(watch, &parent_path);
>  
> @@ -446,6 +454,7 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
>  	*list = &audit_inode_hash[h];
>  error:
>  	path_put(&parent_path);
> +	audit_put_watch(watch);
>  	return ret;
>  }
>  

- RGB

--
Richard Guy Briggs <rgb at redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635