Re: [PATCH 04/11] audit: Embed key into chunk

[Richard Guy Briggs <rgb@redhat.com>]

On 2018-09-04 18:06, Jan Kara wrote:
> Currently chunk hash key (which is in fact pointer to the inode) is
> derived as chunk->mark.conn->obj. It is tricky to make this dereference
> reliable for hash table lookups only under RCU as mark can get detached
> from the connector and connector gets freed independently of the
> running lookup. Thus there is a possible use after free / NULL ptr
> dereference issue:
> 
> CPU1					CPU2
> 					untag_chunk()
> 					  ...
> audit_tree_lookup()
>   list_for_each_entry_rcu(p, list, hash) {
> 					  list_del_rcu(&chunk->hash);
> 					  fsnotify_destroy_mark(entry);
> 					  fsnotify_put_mark(entry)
>     chunk_to_key(p)
>       if (!chunk->mark.connector)
> 					    ...
> 					    hlist_del_init_rcu(&mark->obj_list);
> 					    if (hlist_empty(&conn->list)) {
> 					      inode = fsnotify_detach_connector_from_object(conn);
> 					    mark->connector = NULL;
> 					    ...
> 					    frees connector from workqueue
>       chunk->mark.connector->obj
> 
> This race is probably impossible to hit in practice as the race window
> on CPU1 is very narrow and CPU2 has a lot of code to execute. Still it's
> better to have this fixed. Since the inode the chunk is attached to is
> constant during chunk's lifetime it is easy to cache the key in the
> chunk itself and thus avoid these issues.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  kernel/audit_tree.c | 27 ++++++++-------------------
>  1 file changed, 8 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index c194dbd53753..bac5dd90c629 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -24,6 +24,7 @@ struct audit_tree {
>  
>  struct audit_chunk {
>  	struct list_head hash;
> +	unsigned long key;
>  	struct fsnotify_mark mark;
>  	struct list_head trees;		/* with root here */
>  	int dead;
> @@ -172,21 +173,6 @@ static unsigned long inode_to_key(const struct inode *inode)
>  	return (unsigned long)&inode->i_fsnotify_marks;
>  }
>  
> -/*
> - * Function to return search key in our hash from chunk. Key 0 is special and
> - * should never be present in the hash.
> - */
> -static unsigned long chunk_to_key(struct audit_chunk *chunk)
> -{
> -	/*
> -	 * We have a reference to the mark so it should be attached to a
> -	 * connector.
> -	 */
> -	if (WARN_ON_ONCE(!chunk->mark.connector))
> -		return 0;
> -	return (unsigned long)chunk->mark.connector->obj;
> -}
> -
>  static inline struct list_head *chunk_hash(unsigned long key)
>  {
>  	unsigned long n = key / L1_CACHE_BYTES;
> @@ -196,12 +182,12 @@ static inline struct list_head *chunk_hash(unsigned long key)
>  /* hash_lock & entry->group->mark_mutex is held by caller */
>  static void insert_hash(struct audit_chunk *chunk)
>  {
> -	unsigned long key = chunk_to_key(chunk);
>  	struct list_head *list;
>  
>  	if (!(chunk->mark.flags & FSNOTIFY_MARK_FLAG_ATTACHED))
>  		return;
> -	list = chunk_hash(key);
> +	WARN_ON_ONCE(!chunk->key);
> +	list = chunk_hash(chunk->key);
>  	list_add_rcu(&chunk->hash, list);
>  }
>  
> @@ -213,7 +199,7 @@ struct audit_chunk *audit_tree_lookup(const struct inode *inode)
>  	struct audit_chunk *p;
>  
>  	list_for_each_entry_rcu(p, list, hash) {
> -		if (chunk_to_key(p) == key) {
> +		if (p->key == key) {
>  			atomic_long_inc(&p->refs);
>  			return p;
>  		}
> @@ -295,6 +281,7 @@ static void untag_chunk(struct node *p)
>  
>  	chunk->dead = 1;
>  	spin_lock(&hash_lock);
> +	new->key = chunk->key;
>  	list_replace_init(&chunk->trees, &new->trees);
>  	if (owner->root == chunk) {
>  		list_del_init(&owner->same_root);
> @@ -380,6 +367,7 @@ static int create_chunk(struct inode *inode, struct audit_tree *tree)
>  		tree->root = chunk;
>  		list_add(&tree->same_root, &chunk->trees);
>  	}
> +	chunk->key = inode_to_key(inode);

Is there a patch missing that somehow converts from chunk_to_key() to
inode_to_key() and from chunk->mark.connector->obj to
chunk->mark.connector->inode that I've missed?

Yes.  36f10f55ff1d <amir73il@gmail.com> 2018-06-23 ("fsnotify: let
connector point to an abstract object").  I was looking at audit/next
rather than v4.19-rc1.

>  	insert_hash(chunk);
>  	spin_unlock(&hash_lock);
>  	mutex_unlock(&audit_tree_group->mark_mutex);
> @@ -456,6 +444,7 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
>  		fsnotify_put_mark(old_entry);
>  		return 0;
>  	}
> +	chunk->key = old->key;
>  	list_replace_init(&old->trees, &chunk->trees);
>  	for (n = 0, p = chunk->owners; n < old->count; n++, p++) {
>  		struct audit_tree *s = old->owners[n].owner;
> @@ -654,7 +643,7 @@ void audit_trim_trees(void)
>  			/* this could be NULL if the watch is dying else where... */
>  			node->index |= 1U<<31;
>  			if (iterate_mounts(compare_root,
> -					   (void *)chunk_to_key(chunk),
> +					   (void *)(chunk->key),
>  					   root_mnt))
>  				node->index &= ~(1U<<31);
>  		}
> -- 
> 2.16.4

- RGB

--
Richard Guy Briggs <rgb@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