Commit message: assoc_array: Fix a buggy node-splitting case

This fixes CVE-2017-12193.

Fix a case in the assoc_array implementation in which a new leaf is
added that needs to go into a node that happens to be full, where the
existing leaves in that node cluster together at that level to the
exclusion of new leaf.

What needs to happen is that the existing leaves get moved out to a new
node, N1, at level + 1 and the existing node needs replacing with one,
N0, that has pointers to the new leaf and to N1.

The code that tries to do this gets this wrong in two ways:

 (1) The pointer that should've pointed from N0 to N1 is set to point
     recursively to N0 instead.

 (2) The backpointer from N0 needs to be set correctly in the case N0 is
     either the root node or reached through a shortcut.

Fix this by removing this path and using the split_node path instead,
which achieves the same end, but in a more general way (thanks to Eric
Biggers for spotting the redundancy).

The problem manifests itself as:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
  IP: assoc_array_apply_edit+0x59/0xe5

Fixes: 3cb989501c26 ("Add a generic associative array implementation.")
Reported-and-tested-by: WU Fan <u3536072@connect.hku.hk>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org [v3.13-rc1+]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2017-12193
CWE: CWE-476
Vulnerability description: The assoc_array_insert_into_terminal_node function in lib/assoc_array.c in the Linux kernel before 4.13.11 mishandles node splitting, which allows local users to cause a denial of service (NULL pointer dereference and panic) via a crafted application, as demonstrated by the keyring key type, and key addition and link creation operations.

Project: linux
File: assoc_array.c

static bool assoc_array_insert_into_terminal_node(struct assoc_array_edit *edit,
						  const struct assoc_array_ops *ops,
						  const void *index_key,
						  struct assoc_array_walk_result *result)
{
	struct assoc_array_shortcut *shortcut, *new_s0;
	struct assoc_array_node *node, *new_n0, *new_n1, *side;
	struct assoc_array_ptr *ptr;
	unsigned long dissimilarity, base_seg, blank;
	size_t keylen;
	bool have_meta;
	int level, diff;
	int slot, next_slot, free_slot, i, j;

	node	= result->terminal_node.node;
	level	= result->terminal_node.level;
	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = result->terminal_node.slot;

	pr_devel("-->%s()\n", __func__);

	/* We arrived at a node which doesn't have an onward node or shortcut
	 * pointer that we have to follow.  This means that (a) the leaf we
	 * want must go here (either by insertion or replacement) or (b) we
	 * need to split this node and insert in one of the fragments.
	 */
	free_slot = -1;

	/* Firstly, we have to check the leaves in this node to see if there's
	 * a matching one we should replace in place.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		if (!ptr) {
			free_slot = i;
			continue;
		}
		if (assoc_array_ptr_is_leaf(ptr) &&
		    ops->compare_object(assoc_array_ptr_to_leaf(ptr),
					index_key)) {
			pr_devel("replace in slot %d\n", i);
			edit->leaf_p = &node->slots[i];
			edit->dead_leaf = node->slots[i];
			pr_devel("<--%s() = ok [replace]\n", __func__);
			return true;
		}
	}

	/* If there is a free slot in this node then we can just insert the
	 * leaf here.
	 */
	if (free_slot >= 0) {
		pr_devel("insert in free slot %d\n", free_slot);
		edit->leaf_p = &node->slots[free_slot];
		edit->adjust_count_on = node;
		pr_devel("<--%s() = ok [insert]\n", __func__);
		return true;
	}

	/* The node has no spare slots - so we're either going to have to split
	 * it or insert another node before it.
	 *
	 * Whatever, we're going to need at least two new nodes - so allocate
	 * those now.  We may also need a new shortcut, but we deal with that
	 * when we need it.
	 */
	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n0)
		return false;
	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
	new_n1 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n1)
		return false;
	edit->new_meta[1] = assoc_array_node_to_ptr(new_n1);

	/* We need to find out how similar the leaves are. */
	pr_devel("no spare slots\n");
	have_meta = false;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		if (assoc_array_ptr_is_meta(ptr)) {
			edit->segment_cache[i] = 0xff;
			have_meta = true;
			continue;
		}
		base_seg = ops->get_object_key_chunk(
			assoc_array_ptr_to_leaf(ptr), level);
		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
	}

	if (have_meta) {
		pr_devel("have meta\n");
		goto split_node;
	}

	/* The node contains only leaves */
	dissimilarity = 0;
	base_seg = edit->segment_cache[0];
	for (i = 1; i < ASSOC_ARRAY_FAN_OUT; i++)
		dissimilarity |= edit->segment_cache[i] ^ base_seg;

	pr_devel("only leaves; dissimilarity=%lx\n", dissimilarity);

	if ((dissimilarity & ASSOC_ARRAY_FAN_MASK) == 0) {
		/* The old leaves all cluster in the same slot.  We will need
		 * to insert a shortcut if the new node wants to cluster with them.
		 */
 		if ((edit->segment_cache[ASSOC_ARRAY_FAN_OUT] ^ base_seg) == 0)
 			goto all_leaves_cluster_together;
 
		/* Otherwise all the old leaves cluster in the same slot, but
		 * the new leaf wants to go into a different slot - so we
		 * create a new node (n0) to hold the new leaf and a pointer to
		 * a new node (n1) holding all the old leaves.
		 *
		 * This can be done by falling through to the node splitting
		 * path.
 		 */
		pr_devel("present leaves cluster but not new leaf\n");
 	}
 
 split_node:
 	pr_devel("split node\n");
 
	/* We need to split the current node.  The node must contain anything
	 * from a single leaf (in the one leaf case, this leaf will cluster
	 * with the new leaf) and the rest meta-pointers, to all leaves, some
	 * of which may cluster.
	 *
	 * It won't contain the case in which all the current leaves plus the
	 * new leaves want to cluster in the same slot.
 	 *
 	 * We need to expel at least two leaves out of a set consisting of the
	 * leaves in the node and the new leaf.  The current meta pointers can
	 * just be copied as they shouldn't cluster with any of the leaves.
 	 *
 	 * We need a new node (n0) to replace the current one and a new node to
 	 * take the expelled nodes (n1).
	 */
	edit->set[0].to = assoc_array_node_to_ptr(new_n0);
	new_n0->back_pointer = node->back_pointer;
	new_n0->parent_slot = node->parent_slot;
	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
	new_n1->parent_slot = -1; /* Need to calculate this */

do_split_node:
	pr_devel("do_split_node\n");

	new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
	new_n1->nr_leaves_on_branch = 0;

	/* Begin by finding two matching leaves.  There have to be at least two
	 * that match - even if there are meta pointers - because any leaf that
	 * would match a slot with a meta pointer in it must be somewhere
	 * behind that meta pointer and cannot be here.  Further, given N
	 * remaining leaf slots, we now have N+1 leaves to go in them.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		slot = edit->segment_cache[i];
		if (slot != 0xff)
			for (j = i + 1; j < ASSOC_ARRAY_FAN_OUT + 1; j++)
				if (edit->segment_cache[j] == slot)
					goto found_slot_for_multiple_occupancy;
	}
found_slot_for_multiple_occupancy:
	pr_devel("same slot: %x %x [%02x]\n", i, j, slot);
	BUG_ON(i >= ASSOC_ARRAY_FAN_OUT);
	BUG_ON(j >= ASSOC_ARRAY_FAN_OUT + 1);
	BUG_ON(slot >= ASSOC_ARRAY_FAN_OUT);

	new_n1->parent_slot = slot;

	/* Metadata pointers cannot change slot */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
		if (assoc_array_ptr_is_meta(node->slots[i]))
			new_n0->slots[i] = node->slots[i];
		else
			new_n0->slots[i] = NULL;
	BUG_ON(new_n0->slots[slot] != NULL);
	new_n0->slots[slot] = assoc_array_node_to_ptr(new_n1);

	/* Filter the leaf pointers between the new nodes */
	free_slot = -1;
	next_slot = 0;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		if (assoc_array_ptr_is_meta(node->slots[i]))
			continue;
		if (edit->segment_cache[i] == slot) {
			new_n1->slots[next_slot++] = node->slots[i];
			new_n1->nr_leaves_on_branch++;
		} else {
			do {
				free_slot++;
			} while (new_n0->slots[free_slot] != NULL);
			new_n0->slots[free_slot] = node->slots[i];
		}
	}

	pr_devel("filtered: f=%x n=%x\n", free_slot, next_slot);

	if (edit->segment_cache[ASSOC_ARRAY_FAN_OUT] != slot) {
		do {
			free_slot++;
		} while (new_n0->slots[free_slot] != NULL);
		edit->leaf_p = &new_n0->slots[free_slot];
		edit->adjust_count_on = new_n0;
	} else {
		edit->leaf_p = &new_n1->slots[next_slot++];
		edit->adjust_count_on = new_n1;
	}

	BUG_ON(next_slot <= 1);

	edit->set_backpointers_to = assoc_array_node_to_ptr(new_n0);
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		if (edit->segment_cache[i] == 0xff) {
			ptr = node->slots[i];
			BUG_ON(assoc_array_ptr_is_leaf(ptr));
			if (assoc_array_ptr_is_node(ptr)) {
				side = assoc_array_ptr_to_node(ptr);
				edit->set_backpointers[i] = &side->back_pointer;
			} else {
				shortcut = assoc_array_ptr_to_shortcut(ptr);
				edit->set_backpointers[i] = &shortcut->back_pointer;
			}
		}
	}

	ptr = node->back_pointer;
	if (!ptr)
		edit->set[0].ptr = &edit->array->root;
	else if (assoc_array_ptr_is_node(ptr))
		edit->set[0].ptr = &assoc_array_ptr_to_node(ptr)->slots[node->parent_slot];
	else
		edit->set[0].ptr = &assoc_array_ptr_to_shortcut(ptr)->next_node;
	edit->excised_meta[0] = assoc_array_node_to_ptr(node);
 	pr_devel("<--%s() = ok [split node]\n", __func__);
 	return true;
 
 all_leaves_cluster_together:
 	/* All the leaves, new and old, want to cluster together in this node
 	 * in the same slot, so we have to replace this node with a shortcut to
	 * skip over the identical parts of the key and then place a pair of
	 * nodes, one inside the other, at the end of the shortcut and
	 * distribute the keys between them.
	 *
	 * Firstly we need to work out where the leaves start diverging as a
	 * bit position into their keys so that we know how big the shortcut
	 * needs to be.
	 *
	 * We only need to make a single pass of N of the N+1 leaves because if
	 * any keys differ between themselves at bit X then at least one of
	 * them must also differ with the base key at bit X or before.
	 */
	pr_devel("all leaves cluster together\n");
	diff = INT_MAX;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		int x = ops->diff_objects(assoc_array_ptr_to_leaf(node->slots[i]),
					  index_key);
		if (x < diff) {
			BUG_ON(x < 0);
			diff = x;
		}
	}
	BUG_ON(diff == INT_MAX);
	BUG_ON(diff < level + ASSOC_ARRAY_LEVEL_STEP);

	keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
	keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

	new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
			 keylen * sizeof(unsigned long), GFP_KERNEL);
	if (!new_s0)
		return false;
	edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s0);

	edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
	new_s0->back_pointer = node->back_pointer;
	new_s0->parent_slot = node->parent_slot;
	new_s0->next_node = assoc_array_node_to_ptr(new_n0);
	new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
	new_n0->parent_slot = 0;
	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
	new_n1->parent_slot = -1; /* Need to calculate this */

	new_s0->skip_to_level = level = diff & ~ASSOC_ARRAY_LEVEL_STEP_MASK;
	pr_devel("skip_to_level = %d [diff %d]\n", level, diff);
	BUG_ON(level <= 0);

	for (i = 0; i < keylen; i++)
		new_s0->index_key[i] =
			ops->get_key_chunk(index_key, i * ASSOC_ARRAY_KEY_CHUNK_SIZE);

	blank = ULONG_MAX << (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
	pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, level, blank);
	new_s0->index_key[keylen - 1] &= ~blank;

	/* This now reduces to a node splitting exercise for which we'll need
	 * to regenerate the disparity table.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		base_seg = ops->get_object_key_chunk(assoc_array_ptr_to_leaf(ptr),
						     level);
		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
	}

	base_seg = ops->get_key_chunk(index_key, level);
	base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = base_seg & ASSOC_ARRAY_FAN_MASK;
	goto do_split_node;
}