static bool search_nested_keyrings(struct key *keyring,
				   struct keyring_search_context *ctx)
{
	struct {
		struct key *keyring;
		struct assoc_array_node *node;
		int slot;
	} stack[KEYRING_SEARCH_MAX_DEPTH];

	struct assoc_array_shortcut *shortcut;
	struct assoc_array_node *node;
	struct assoc_array_ptr *ptr;
	struct key *key;
	int sp = 0, slot;

	kenter("{%d},{%s,%s}",
	       keyring->serial,
	       ctx->index_key.type->name,
	       ctx->index_key.description);

	if (ctx->index_key.description)
		ctx->index_key.desc_len = strlen(ctx->index_key.description);

	/* Check to see if this top-level keyring is what we are looking for
	 * and whether it is valid or not.
	 */
	if (ctx->match_data.lookup_type == KEYRING_SEARCH_LOOKUP_ITERATE ||
	    keyring_compare_object(keyring, &ctx->index_key)) {
		ctx->skipped_ret = 2;
		ctx->flags |= KEYRING_SEARCH_DO_STATE_CHECK;
		switch (ctx->iterator(keyring_key_to_ptr(keyring), ctx)) {
		case 1:
			goto found;
		case 2:
			return false;
		default:
			break;
		}
	}

	ctx->skipped_ret = 0;
	if (ctx->flags & KEYRING_SEARCH_NO_STATE_CHECK)
		ctx->flags &= ~KEYRING_SEARCH_DO_STATE_CHECK;

	/* Start processing a new keyring */
descend_to_keyring:
	kdebug("descend to %d", keyring->serial);
	if (keyring->flags & ((1 << KEY_FLAG_INVALIDATED) |
			      (1 << KEY_FLAG_REVOKED)))
		goto not_this_keyring;

	/* Search through the keys in this keyring before its searching its
	 * subtrees.
	 */
	if (search_keyring(keyring, ctx))
		goto found;

	/* Then manually iterate through the keyrings nested in this one.
	 *
	 * Start from the root node of the index tree.  Because of the way the
	 * hash function has been set up, keyrings cluster on the leftmost
	 * branch of the root node (root slot 0) or in the root node itself.
	 * Non-keyrings avoid the leftmost branch of the root entirely (root
	 * slots 1-15).
	 */
	ptr = ACCESS_ONCE(keyring->keys.root);
	if (!ptr)
		goto not_this_keyring;

	if (assoc_array_ptr_is_shortcut(ptr)) {
		/* If the root is a shortcut, either the keyring only contains
		 * keyring pointers (everything clusters behind root slot 0) or
		 * doesn't contain any keyring pointers.
		 */
		shortcut = assoc_array_ptr_to_shortcut(ptr);
		smp_read_barrier_depends();
		if ((shortcut->index_key[0] & ASSOC_ARRAY_FAN_MASK) != 0)
			goto not_this_keyring;

		ptr = ACCESS_ONCE(shortcut->next_node);
		node = assoc_array_ptr_to_node(ptr);
		goto begin_node;
	}

	node = assoc_array_ptr_to_node(ptr);
	smp_read_barrier_depends();

	ptr = node->slots[0];
	if (!assoc_array_ptr_is_meta(ptr))
		goto begin_node;

descend_to_node:
	/* Descend to a more distal node in this keyring's content tree and go
	 * through that.
	 */
	kdebug("descend");
	if (assoc_array_ptr_is_shortcut(ptr)) {
		shortcut = assoc_array_ptr_to_shortcut(ptr);
		smp_read_barrier_depends();
		ptr = ACCESS_ONCE(shortcut->next_node);
		BUG_ON(!assoc_array_ptr_is_node(ptr));
	}
	node = assoc_array_ptr_to_node(ptr);

begin_node:
	kdebug("begin_node");
	smp_read_barrier_depends();
	slot = 0;
ascend_to_node:
	/* Go through the slots in a node */
	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = ACCESS_ONCE(node->slots[slot]);

		if (assoc_array_ptr_is_meta(ptr) && node->back_pointer)
			goto descend_to_node;

		if (!keyring_ptr_is_keyring(ptr))
			continue;

		key = keyring_ptr_to_key(ptr);

		if (sp >= KEYRING_SEARCH_MAX_DEPTH) {
			if (ctx->flags & KEYRING_SEARCH_DETECT_TOO_DEEP) {
				ctx->result = ERR_PTR(-ELOOP);
				return false;
			}
			goto not_this_keyring;
		}

		/* Search a nested keyring */
		if (!(ctx->flags & KEYRING_SEARCH_NO_CHECK_PERM) &&
		    key_task_permission(make_key_ref(key, ctx->possessed),
					ctx->cred, KEY_NEED_SEARCH) < 0)
			continue;

		/* stack the current position */
		stack[sp].keyring = keyring;
		stack[sp].node = node;
		stack[sp].slot = slot;
		sp++;

		/* begin again with the new keyring */
		keyring = key;
		goto descend_to_keyring;
	}

	/* We've dealt with all the slots in the current node, so now we need
	 * to ascend to the parent and continue processing there.
	 */
	ptr = ACCESS_ONCE(node->back_pointer);
	slot = node->parent_slot;

	if (ptr && assoc_array_ptr_is_shortcut(ptr)) {
		shortcut = assoc_array_ptr_to_shortcut(ptr);
		smp_read_barrier_depends();
		ptr = ACCESS_ONCE(shortcut->back_pointer);
		slot = shortcut->parent_slot;
	}
	if (!ptr)
		goto not_this_keyring;
	node = assoc_array_ptr_to_node(ptr);
	smp_read_barrier_depends();
	slot++;

	/* If we've ascended to the root (zero backpointer), we must have just
	 * finished processing the leftmost branch rather than the root slots -
	 * so there can't be any more keyrings for us to find.
	 */
	if (node->back_pointer) {
		kdebug("ascend %d", slot);
		goto ascend_to_node;
	}

	/* The keyring we're looking at was disqualified or didn't contain a
	 * matching key.
	 */
not_this_keyring:
	kdebug("not_this_keyring %d", sp);
	if (sp <= 0) {
		kleave(" = false");
		return false;
	}

	/* Resume the processing of a keyring higher up in the tree */
	sp--;
	keyring = stack[sp].keyring;
	node = stack[sp].node;
	slot = stack[sp].slot + 1;
	kdebug("ascend to %d [%d]", keyring->serial, slot);
	goto ascend_to_node;

	/* We found a viable match */
found:
	key = key_ref_to_ptr(ctx->result);
	key_check(key);
	if (!(ctx->flags & KEYRING_SEARCH_NO_UPDATE_TIME)) {
		key->last_used_at = ctx->now.tv_sec;
		keyring->last_used_at = ctx->now.tv_sec;
		while (sp > 0)
			stack[--sp].keyring->last_used_at = ctx->now.tv_sec;
	}
	kleave(" = true");
	return true;
}

The product dereferences a pointer that it expects to be valid but is NULL.