Re: [PATCH 5/6] audit: Make hash table insertion safe against concurrent lookups

Amir Goldstein
2018-06-29T13:02:10Z

On Thu, Jun 28, 2018 at 7:40 PM, Jan Kara <jack@suse.cz> wrote:
> Currently, the audit tree code does not make sure that when a chunk in
> inserted into the hash table, it is fully initialized. So in theory (in
> practice only on DEC Alpha) a user of RCU lookup could see uninitialized
> structure in the hash table and crash. Add appropriate barriers between
> initialization of the structure and its insertion into hash table.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> kernel/audit_tree.c | 28 +++++++++++++++++++++++++---
> 1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index 89ad8857a578..46cc6d046c75 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -198,7 +198,11 @@ struct audit_chunk *audit_tree_lookup(const struct inode *inode)
> struct audit_chunk *p;
>
> list_for_each_entry_rcu(p, list, hash) {
> - if (p->key == key) {
> + /*
> + * We use a data dependency barrier in READ_ONCE() to make sure
> + * the chunk we see is fully initialized.
> + */
> + if (READ_ONCE(p->key) == key) {
> atomic_long_inc(&p->refs);
> return p;
> }
> @@ -303,9 +307,15 @@ static void untag_chunk(struct node *p)
> list_replace_init(&chunk->owners[j].list, &new->owners[i].list);
> }
>
> - list_replace_rcu(&chunk->hash, &new->hash);
> list_for_each_entry(owner, &new->trees, same_root)
> owner->root = new;
> + /*
> + * Make sure chunk is fully initialized before making it visible in the
> + * hash. Pairs with a data dependency barrier in READ_ONCE() in
> + * audit_tree_lookup().
> + */
> + smp_wmb();
> + list_replace_rcu(&chunk->hash, &new->hash);
> spin_unlock(&hash_lock);
> fsnotify_detach_mark(entry);
> mutex_unlock(&entry->group->mark_mutex);
> @@ -367,6 +377,12 @@ static int create_chunk(struct inode *inode, struct audit_tree *tree)
> list_add(&tree->same_root, &chunk->trees);
> }
> chunk->key = inode_to_key(inode);
> + /*
> + * Make sure chunk is fully initialized before making it visible in the
> + * hash. Pairs with a data dependency barrier in READ_ONCE() in
> + * audit_tree_lookup().
> + */
> + smp_wmb();
> insert_hash(chunk);
> spin_unlock(&hash_lock);
> mutex_unlock(&audit_tree_group->mark_mutex);
> @@ -458,7 +474,6 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
> p->owner = tree;
> get_tree(tree);
> list_add(&p->list, &tree->chunks);
> - list_replace_rcu(&old->hash, &chunk->hash);
> list_for_each_entry(owner, &chunk->trees, same_root)
> owner->root = chunk;
> old->dead = 1;
> @@ -466,6 +481,13 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
> tree->root = chunk;
> list_add(&tree->same_root, &chunk->trees);
> }
> + /*
> + * Make sure chunk is fully initialized before making it visible in the
> + * hash. Pairs with a data dependency barrier in READ_ONCE() in
> + * audit_tree_lookup().
> + */
> + smp_wmb();
> + list_replace_rcu(&old->hash, &chunk->hash);

IMO, now that list_replace_rcu() is no longer a one liner (including the wmb and
comment above) it would be cleaner to have a helper update_hash(old, chunk)
right next to insert_hash() and for the same reason smp_wmb with the comment
should go into insert_hash() helpler.

Thanks,
Amir.