buttox/security/keys/keyring.c

/* Keyring handling
 *
 * Copyright (C) 2004-2005, 2008, 2013 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 */

#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/security.h>
#include <linux/seq_file.h>
#include <linux/err.h>
#include <keys/keyring-type.h>
#include <keys/user-type.h>
#include <linux/assoc_array_priv.h>
#include <linux/uaccess.h>
#include "internal.h"

/*
 * When plumbing the depths of the key tree, this sets a hard limit
 * set on how deep we're willing to go.
 */
#define KEYRING_SEARCH_MAX_DEPTH 6

/*
 * We keep all named keyrings in a hash to speed looking them up.
 */
#define KEYRING_NAME_HASH_SIZE	(1 << 5)

/*
 * We mark pointers we pass to the associative array with bit 1 set if
 * they're keyrings and clear otherwise.
 */
#define KEYRING_PTR_SUBTYPE	0x2UL

static inline bool keyring_ptr_is_keyring(const struct assoc_array_ptr *x)
{
	return (unsigned long)x & KEYRING_PTR_SUBTYPE;
}
static inline struct key *keyring_ptr_to_key(const struct assoc_array_ptr *x)
{
	void *object = assoc_array_ptr_to_leaf(x);
	return (struct key *)((unsigned long)object & ~KEYRING_PTR_SUBTYPE);
}
static inline void *keyring_key_to_ptr(struct key *key)
{
	if (key->type == &key_type_keyring)
		return (void *)((unsigned long)key | KEYRING_PTR_SUBTYPE);
	return key;
}

static struct list_head	keyring_name_hash[KEYRING_NAME_HASH_SIZE];
static DEFINE_RWLOCK(keyring_name_lock);

static inline unsigned keyring_hash(const char *desc)
{
	unsigned bucket = 0;

	for (; *desc; desc++)
		bucket += (unsigned char)*desc;

	return bucket & (KEYRING_NAME_HASH_SIZE - 1);
}

/*
 * The keyring key type definition.  Keyrings are simply keys of this type and
 * can be treated as ordinary keys in addition to having their own special
 * operations.
 */
static int keyring_preparse(struct key_preparsed_payload *prep);
static void keyring_free_preparse(struct key_preparsed_payload *prep);
static int keyring_instantiate(struct key *keyring,
			       struct key_preparsed_payload *prep);
static void keyring_revoke(struct key *keyring);
static void keyring_destroy(struct key *keyring);
static void keyring_describe(const struct key *keyring, struct seq_file *m);
static long keyring_read(const struct key *keyring,
			 char __user *buffer, size_t buflen);

struct key_type key_type_keyring = {
	.name		= "keyring",
	.def_datalen	= 0,
	.preparse	= keyring_preparse,
	.free_preparse	= keyring_free_preparse,
	.instantiate	= keyring_instantiate,
	.revoke		= keyring_revoke,
	.destroy	= keyring_destroy,
	.describe	= keyring_describe,
	.read		= keyring_read,
};
EXPORT_SYMBOL(key_type_keyring);

/*
 * Semaphore to serialise link/link calls to prevent two link calls in parallel
 * introducing a cycle.
 */
static DECLARE_RWSEM(keyring_serialise_link_sem);

/*
 * Publish the name of a keyring so that it can be found by name (if it has
 * one).
 */
static void keyring_publish_name(struct key *keyring)
{
	int bucket;

	if (keyring->description) {
		bucket = keyring_hash(keyring->description);

		write_lock(&keyring_name_lock);

		if (!keyring_name_hash[bucket].next)
			INIT_LIST_HEAD(&keyring_name_hash[bucket]);

		list_add_tail(&keyring->name_link,
			      &keyring_name_hash[bucket]);

		write_unlock(&keyring_name_lock);
	}
}

/*
 * Preparse a keyring payload
 */
static int keyring_preparse(struct key_preparsed_payload *prep)
{
	return prep->datalen != 0 ? -EINVAL : 0;
}

/*
 * Free a preparse of a user defined key payload
 */
static void keyring_free_preparse(struct key_preparsed_payload *prep)
{
}

/*
 * Initialise a keyring.
 *
 * Returns 0 on success, -EINVAL if given any data.
 */
static int keyring_instantiate(struct key *keyring,
			       struct key_preparsed_payload *prep)
{
	assoc_array_init(&keyring->keys);
	/* make the keyring available by name if it has one */
	keyring_publish_name(keyring);
	return 0;
}

/*
 * Multiply 64-bits by 32-bits to 96-bits and fold back to 64-bit.  Ideally we'd
 * fold the carry back too, but that requires inline asm.
 */
static u64 mult_64x32_and_fold(u64 x, u32 y)
{
	u64 hi = (u64)(u32)(x >> 32) * y;
	u64 lo = (u64)(u32)(x) * y;
	return lo + ((u64)(u32)hi << 32) + (u32)(hi >> 32);
}

/*
 * Hash a key type and description.
 */
static unsigned long hash_key_type_and_desc(const struct keyring_index_key *index_key)
{
	const unsigned level_shift = ASSOC_ARRAY_LEVEL_STEP;
	const unsigned long fan_mask = ASSOC_ARRAY_FAN_MASK;
	const char *description = index_key->description;
	unsigned long hash, type;
	u32 piece;
	u64 acc;
	int n, desc_len = index_key->desc_len;

	type = (unsigned long)index_key->type;

	acc = mult_64x32_and_fold(type, desc_len + 13);
	acc = mult_64x32_and_fold(acc, 9207);
	for (;;) {
		n = desc_len;
		if (n <= 0)
			break;
		if (n > 4)
			n = 4;
		piece = 0;
		memcpy(&piece, description, n);
		description += n;
		desc_len -= n;
		acc = mult_64x32_and_fold(acc, piece);
		acc = mult_64x32_and_fold(acc, 9207);
	}

	/* Fold the hash down to 32 bits if need be. */
	hash = acc;
	if (ASSOC_ARRAY_KEY_CHUNK_SIZE == 32)
		hash ^= acc >> 32;

	/* Squidge all the keyrings into a separate part of the tree to
	 * ordinary keys by making sure the lowest level segment in the hash is
	 * zero for keyrings and non-zero otherwise.
	 */
	if (index_key->type != &key_type_keyring && (hash & fan_mask) == 0)
		return hash | (hash >> (ASSOC_ARRAY_KEY_CHUNK_SIZE - level_shift)) | 1;
	if (index_key->type == &key_type_keyring && (hash & fan_mask) != 0)
		return (hash + (hash << level_shift)) & ~fan_mask;
	return hash;
}

/*
 * Build the next index key chunk.
 *
 * On 32-bit systems the index key is laid out as:
 *
 *	0	4	5	9...
 *	hash	desclen	typeptr	desc[]
 *
 * On 64-bit systems:
 *
 *	0	8	9	17...
 *	hash	desclen	typeptr	desc[]
 *
 * We return it one word-sized chunk at a time.
 */
static unsigned long keyring_get_key_chunk(const void *data, int level)
{
	const struct keyring_index_key *index_key = data;
	unsigned long chunk = 0;
	long offset = 0;
	int desc_len = index_key->desc_len, n = sizeof(chunk);

	level /= ASSOC_ARRAY_KEY_CHUNK_SIZE;
	switch (level) {
	case 0:
		return hash_key_type_and_desc(index_key);
	case 1:
		return ((unsigned long)index_key->type << 8) | desc_len;
	case 2:
		if (desc_len == 0)
			return (u8)((unsigned long)index_key->type >>
				    (ASSOC_ARRAY_KEY_CHUNK_SIZE - 8));
		n--;
		offset = 1;
	default:
		offset += sizeof(chunk) - 1;
		offset += (level - 3) * sizeof(chunk);
		if (offset >= desc_len)
			return 0;
		desc_len -= offset;
		if (desc_len > n)
			desc_len = n;
		offset += desc_len;
		do {
			chunk <<= 8;
			chunk |= ((u8*)index_key->description)[--offset];
		} while (--desc_len > 0);

		if (level == 2) {
			chunk <<= 8;
			chunk |= (u8)((unsigned long)index_key->type >>
				      (ASSOC_ARRAY_KEY_CHUNK_SIZE - 8));
		}
		return chunk;
	}
}

static unsigned long keyring_get_object_key_chunk(const void *object, int level)
{
	const struct key *key = keyring_ptr_to_key(object);
	return keyring_get_key_chunk(&key->index_key, level);
}

static bool keyring_compare_object(const void *object, const void *data)
{
	const struct keyring_index_key *index_key = data;
	const struct key *key = keyring_ptr_to_key(object);

	return key->index_key.type == index_key->type &&
		key->index_key.desc_len == index_key->desc_len &&
		memcmp(key->index_key.description, index_key->description,
		       index_key->desc_len) == 0;
}

/*
 * Compare the index keys of a pair of objects and determine the bit position
 * at which they differ - if they differ.
 */
static int keyring_diff_objects(const void *object, const void *data)
{
	const struct key *key_a = keyring_ptr_to_key(object);
	const struct keyring_index_key *a = &key_a->index_key;
	const struct keyring_index_key *b = data;
	unsigned long seg_a, seg_b;
	int level, i;

	level = 0;
	seg_a = hash_key_type_and_desc(a);
	seg_b = hash_key_type_and_desc(b);
	if ((seg_a ^ seg_b) != 0)
		goto differ;

	/* The number of bits contributed by the hash is controlled by a
	 * constant in the assoc_array headers.  Everything else thereafter we
	 * can deal with as being machine word-size dependent.
	 */
	level += ASSOC_ARRAY_KEY_CHUNK_SIZE / 8;
	seg_a = a->desc_len;
	seg_b = b->desc_len;
	if ((seg_a ^ seg_b) != 0)
		goto differ;

	/* The next bit may not work on big endian */
	level++;
	seg_a = (unsigned long)a->type;
	seg_b = (unsigned long)b->type;
	if ((seg_a ^ seg_b) != 0)
		goto differ;

	level += sizeof(unsigned long);
	if (a->desc_len == 0)
		goto same;

	i = 0;
	if (((unsigned long)a->description | (unsigned long)b->description) &
	    (sizeof(unsigned long) - 1)) {
		do {
			seg_a = *(unsigned long *)(a->description + i);
			seg_b = *(unsigned long *)(b->description + i);
			if ((seg_a ^ seg_b) != 0)
				goto differ_plus_i;
			i += sizeof(unsigned long);
		} while (i < (a->desc_len & (sizeof(unsigned long) - 1)));
	}

	for (; i < a->desc_len; i++) {
		seg_a = *(unsigned char *)(a->description + i);
		seg_b = *(unsigned char *)(b->description + i);
		if ((seg_a ^ seg_b) != 0)
			goto differ_plus_i;
	}

same:
	return -1;

differ_plus_i:
	level += i;
differ:
	i = level * 8 + __ffs(seg_a ^ seg_b);
	return i;
}

/*
 * Free an object after stripping the keyring flag off of the pointer.
 */
static void keyring_free_object(void *object)
{
	key_put(keyring_ptr_to_key(object));
}

/*
 * Operations for keyring management by the index-tree routines.
 */
static const struct assoc_array_ops keyring_assoc_array_ops = {
	.get_key_chunk		= keyring_get_key_chunk,
	.get_object_key_chunk	= keyring_get_object_key_chunk,
	.compare_object		= keyring_compare_object,
	.diff_objects		= keyring_diff_objects,
	.free_object		= keyring_free_object,
};

/*
 * Clean up a keyring when it is destroyed.  Unpublish its name if it had one
 * and dispose of its data.
 *
 * The garbage collector detects the final key_put(), removes the keyring from
 * the serial number tree and then does RCU synchronisation before coming here,
 * so we shouldn't need to worry about code poking around here with the RCU
 * readlock held by this time.
 */
static void keyring_destroy(struct key *keyring)
{
	if (keyring->description) {
		write_lock(&keyring_name_lock);

		if (keyring->name_link.next != NULL &&
		    !list_empty(&keyring->name_link))
			list_del(&keyring->name_link);

		write_unlock(&keyring_name_lock);
	}

	assoc_array_destroy(&keyring->keys, &keyring_assoc_array_ops);
}

/*
 * Describe a keyring for /proc.
 */
static void keyring_describe(const struct key *keyring, struct seq_file *m)
{
	if (keyring->description)
		seq_puts(m, keyring->description);
	else
		seq_puts(m, "[anon]");

	if (key_is_positive(keyring)) {
		if (keyring->keys.nr_leaves_on_tree != 0)
			seq_printf(m, ": %lu", keyring->keys.nr_leaves_on_tree);
		else
			seq_puts(m, ": empty");
	}
}

struct keyring_read_iterator_context {
	size_t			buflen;
	size_t			count;
	key_serial_t __user	*buffer;
};

static int keyring_read_iterator(const void *object, void *data)
{
	struct keyring_read_iterator_context *ctx = data;
	const struct key *key = keyring_ptr_to_key(object);
	int ret;

	kenter("{%s,%d},,{%zu/%zu}",
	       key->type->name, key->serial, ctx->count, ctx->buflen);

	if (ctx->count >= ctx->buflen)
		return 1;

	ret = put_user(key->serial, ctx->buffer);
	if (ret < 0)
		return ret;
	ctx->buffer++;
	ctx->count += sizeof(key->serial);
	return 0;
}

/*
 * Read a list of key IDs from the keyring's contents in binary form
 *
 * The keyring's semaphore is read-locked by the caller.  This prevents someone
 * from modifying it under us - which could cause us to read key IDs multiple
 * times.
 */
static long keyring_read(const struct key *keyring,
			 char __user *buffer, size_t buflen)
{
	struct keyring_read_iterator_context ctx;
	long ret;

	kenter("{%d},,%zu", key_serial(keyring), buflen);

	if (buflen & (sizeof(key_serial_t) - 1))
		return -EINVAL;

	/* Copy as many key IDs as fit into the buffer */
	if (buffer && buflen) {
		ctx.buffer = (key_serial_t __user *)buffer;
		ctx.buflen = buflen;
		ctx.count = 0;
		ret = assoc_array_iterate(&keyring->keys,
					  keyring_read_iterator, &ctx);
		if (ret < 0) {
			kleave(" = %ld [iterate]", ret);
			return ret;
		}
	}

	/* Return the size of the buffer needed */
	ret = keyring->keys.nr_leaves_on_tree * sizeof(key_serial_t);
	if (ret <= buflen)
		kleave("= %ld [ok]", ret);
	else
		kleave("= %ld [buffer too small]", ret);
	return ret;
}

/*
 * Allocate a keyring and link into the destination keyring.
 */
struct key *keyring_alloc(const char *description, kuid_t uid, kgid_t gid,
			  const struct cred *cred, key_perm_t perm,
			  unsigned long flags,
			  int (*restrict_link)(struct key *,
					       const struct key_type *,
					       const union key_payload *),
			  struct key *dest)
{
	struct key *keyring;
	int ret;

	keyring = key_alloc(&key_type_keyring, description,
			    uid, gid, cred, perm, flags, restrict_link);
	if (!IS_ERR(keyring)) {
		ret = key_instantiate_and_link(keyring, NULL, 0, dest, NULL);
		if (ret < 0) {
			key_put(keyring);
			keyring = ERR_PTR(ret);
		}
	}

	return keyring;
}
EXPORT_SYMBOL(keyring_alloc);

/**
 * restrict_link_reject - Give -EPERM to restrict link
 * @keyring: The keyring being added to.
 * @type: The type of key being added.
 * @payload: The payload of the key intended to be added.
 *
 * Reject the addition of any links to a keyring.  It can be overridden by
 * passing KEY_ALLOC_BYPASS_RESTRICTION to key_instantiate_and_link() when
 * adding a key to a keyring.
 *
 * This is meant to be passed as the restrict_link parameter to
 * keyring_alloc().
 */
int restrict_link_reject(struct key *keyring,
			 const struct key_type *type,
			 const union key_payload *payload)
{
	return -EPERM;
}

/*
 * By default, we keys found by getting an exact match on their descriptions.
 */
bool key_default_cmp(const struct key *key,
		     const struct key_match_data *match_data)
{
	return strcmp(key->description, match_data->raw_data) == 0;
}

/*
 * Iteration function to consider each key found.
 */
static int keyring_search_iterator(const void *object, void *iterator_data)
{
	struct keyring_search_context *ctx = iterator_data;
	const struct key *key = keyring_ptr_to_key(object);
	unsigned long kflags = READ_ONCE(key->flags);
	short state = READ_ONCE(key->state);

	kenter("{%d}", key->serial);

	/* ignore keys not of this type */
	if (key->type != ctx->index_key.type) {
		kleave(" = 0 [!type]");
		return 0;
	}

	/* skip invalidated, revoked and expired keys */
	if (ctx->flags & KEYRING_SEARCH_DO_STATE_CHECK) {
		if (kflags & ((1 << KEY_FLAG_INVALIDATED) |
			      (1 << KEY_FLAG_REVOKED))) {
			ctx->result = ERR_PTR(-EKEYREVOKED);
			kleave(" = %d [invrev]", ctx->skipped_ret);
			goto skipped;
		}

		if (key->expiry && ctx->now.tv_sec >= key->expiry) {
			if (!(ctx->flags & KEYRING_SEARCH_SKIP_EXPIRED))
				ctx->result = ERR_PTR(-EKEYEXPIRED);
			kleave(" = %d [expire]", ctx->skipped_ret);
			goto skipped;
		}
	}

	/* keys that don't match */
	if (!ctx->match_data.cmp(key, &ctx->match_data)) {
		kleave(" = 0 [!match]");
		return 0;
	}

	/* key must have search permissions */
	if (!(ctx->flags & KEYRING_SEARCH_NO_CHECK_PERM) &&
	    key_task_permission(make_key_ref(key, ctx->possessed),
				ctx->cred, KEY_NEED_SEARCH) < 0) {
		ctx->result = ERR_PTR(-EACCES);
		kleave(" = %d [!perm]", ctx->skipped_ret);
		goto skipped;
	}

	if (ctx->flags & KEYRING_SEARCH_DO_STATE_CHECK) {
		/* we set a different error code if we pass a negative key */
		if (state < 0) {
			ctx->result = ERR_PTR(state);
			kleave(" = %d [neg]", ctx->skipped_ret);
			goto skipped;
		}
	}

	/* Found */
	ctx->result = make_key_ref(key, ctx->possessed);
	kleave(" = 1 [found]");
	return 1;

skipped:
	return ctx->skipped_ret;
}

/*
 * Search inside a keyring for a key.  We can search by walking to it
 * directly based on its index-key or we can iterate over the entire
 * tree looking for it, based on the match function.
 */
static int search_keyring(struct key *keyring, struct keyring_search_context *ctx)
{
	if (ctx->match_data.lookup_type == KEYRING_SEARCH_LOOKUP_DIRECT) {
		const void *object;

		object = assoc_array_find(&keyring->keys,
					  &keyring_assoc_array_ops,
					  &ctx->index_key);
		return object ? ctx->iterator(object, ctx) : 0;
	}
	return assoc_array_iterate(&keyring->keys, ctx->iterator, ctx);
}

/*
 * Search a tree of keyrings that point to other keyrings up to the maximum
 * depth.
 */
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);

#define STATE_CHECKS (KEYRING_SEARCH_NO_STATE_CHECK | KEYRING_SEARCH_DO_STATE_CHECK)
	BUG_ON((ctx->flags & STATE_CHECKS) == 0 ||
	       (ctx->flags & STATE_CHECKS) == STATE_CHECKS);

	/* 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;
		switch (ctx->iterator(keyring_key_to_ptr(keyring), ctx)) {
		case 1:
			goto found;
		case 2:
			return false;
		default:
			break;
		}
	}

	ctx->skipped_ret = 0;

	/* 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;
}

/**
 * keyring_search_aux - Search a keyring tree for a key matching some criteria
 * @keyring_ref: A pointer to the keyring with possession indicator.
 * @ctx: The keyring search context.
 *
 * Search the supplied keyring tree for a key that matches the criteria given.
 * The root keyring and any linked keyrings must grant Search permission to the
 * caller to be searchable and keys can only be found if they too grant Search
 * to the caller. The possession flag on the root keyring pointer controls use
 * of the possessor bits in permissions checking of the entire tree.  In
 * addition, the LSM gets to forbid keyring searches and key matches.
 *
 * The search is performed as a breadth-then-depth search up to the prescribed
 * limit (KEYRING_SEARCH_MAX_DEPTH).
 *
 * Keys are matched to the type provided and are then filtered by the match
 * function, which is given the description to use in any way it sees fit.  The
 * match function may use any attributes of a key that it wishes to to
 * determine the match.  Normally the match function from the key type would be
 * used.
 *
 * RCU can be used to prevent the keyring key lists from disappearing without
 * the need to take lots of locks.
 *
 * Returns a pointer to the found key and increments the key usage count if
 * successful; -EAGAIN if no matching keys were found, or if expired or revoked
 * keys were found; -ENOKEY if only negative keys were found; -ENOTDIR if the
 * specified keyring wasn't a keyring.
 *
 * In the case of a successful return, the possession attribute from
 * @keyring_ref is propagated to the returned key reference.
 */
key_ref_t keyring_search_aux(key_ref_t keyring_ref,
			     struct keyring_search_context *ctx)
{
	struct key *keyring;
	long err;

	ctx->iterator = keyring_search_iterator;
	ctx->possessed = is_key_possessed(keyring_ref);
	ctx->result = ERR_PTR(-EAGAIN);

	keyring = key_ref_to_ptr(keyring_ref);
	key_check(keyring);

	if (keyring->type != &key_type_keyring)
		return ERR_PTR(-ENOTDIR);

	if (!(ctx->flags & KEYRING_SEARCH_NO_CHECK_PERM)) {
		err = key_task_permission(keyring_ref, ctx->cred, KEY_NEED_SEARCH);
		if (err < 0)
			return ERR_PTR(err);
	}

	rcu_read_lock();
	ctx->now = current_kernel_time();
	if (search_nested_keyrings(keyring, ctx))
		__key_get(key_ref_to_ptr(ctx->result));
	rcu_read_unlock();
	return ctx->result;
}

/**
 * keyring_search - Search the supplied keyring tree for a matching key
 * @keyring: The root of the keyring tree to be searched.
 * @type: The type of keyring we want to find.
 * @description: The name of the keyring we want to find.
 *
 * As keyring_search_aux() above, but using the current task's credentials and
 * type's default matching function and preferred search method.
 */
key_ref_t keyring_search(key_ref_t keyring,
			 struct key_type *type,
			 const char *description)
{
	struct keyring_search_context ctx = {
		.index_key.type		= type,
		.index_key.description	= description,
		.index_key.desc_len	= strlen(description),
		.cred			= current_cred(),
		.match_data.cmp		= key_default_cmp,
		.match_data.raw_data	= description,
		.match_data.lookup_type	= KEYRING_SEARCH_LOOKUP_DIRECT,
		.flags			= KEYRING_SEARCH_DO_STATE_CHECK,
	};
	key_ref_t key;
	int ret;

	if (type->match_preparse) {
		ret = type->match_preparse(&ctx.match_data);
		if (ret < 0)
			return ERR_PTR(ret);
	}

	key = keyring_search_aux(keyring, &ctx);

	if (type->match_free)
		type->match_free(&ctx.match_data);
	return key;
}
EXPORT_SYMBOL(keyring_search);

/*
 * Search the given keyring for a key that might be updated.
 *
 * The caller must guarantee that the keyring is a keyring and that the
 * permission is granted to modify the keyring as no check is made here.  The
 * caller must also hold a lock on the keyring semaphore.
 *
 * Returns a pointer to the found key with usage count incremented if
 * successful and returns NULL if not found.  Revoked and invalidated keys are
 * skipped over.
 *
 * If successful, the possession indicator is propagated from the keyring ref
 * to the returned key reference.
 */
key_ref_t find_key_to_update(key_ref_t keyring_ref,
			     const struct keyring_index_key *index_key)
{
	struct key *keyring, *key;
	const void *object;

	keyring = key_ref_to_ptr(keyring_ref);

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

	object = assoc_array_find(&keyring->keys, &keyring_assoc_array_ops,
				  index_key);

	if (object)
		goto found;

	kleave(" = NULL");
	return NULL;

found:
	key = keyring_ptr_to_key(object);
	if (key->flags & ((1 << KEY_FLAG_INVALIDATED) |
			  (1 << KEY_FLAG_REVOKED))) {
		kleave(" = NULL [x]");
		return NULL;
	}
	__key_get(key);
	kleave(" = {%d}", key->serial);
	return make_key_ref(key, is_key_possessed(keyring_ref));
}

/*
 * Find a keyring with the specified name.
 *
 * Only keyrings that have nonzero refcount, are not revoked, and are owned by a
 * user in the current user namespace are considered.  If @uid_keyring is %true,
 * the keyring additionally must have been allocated as a user or user session
 * keyring; otherwise, it must grant Search permission directly to the caller.
 *
 * Returns a pointer to the keyring with the keyring's refcount having being
 * incremented on success.  -ENOKEY is returned if a key could not be found.
 */
struct key *find_keyring_by_name(const char *name, bool uid_keyring)
{
	struct key *keyring;
	int bucket;

	if (!name)
		return ERR_PTR(-EINVAL);

	bucket = keyring_hash(name);

	read_lock(&keyring_name_lock);

	if (keyring_name_hash[bucket].next) {
		/* search this hash bucket for a keyring with a matching name
		 * that's readable and that hasn't been revoked */
		list_for_each_entry(keyring,
				    &keyring_name_hash[bucket],
				    name_link
				    ) {
			if (!kuid_has_mapping(current_user_ns(), keyring->user->uid))
				continue;

			if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
				continue;

			if (strcmp(keyring->description, name) != 0)
				continue;

			if (uid_keyring) {
				if (!test_bit(KEY_FLAG_UID_KEYRING,
					      &keyring->flags))
					continue;
			} else {
				if (key_permission(make_key_ref(keyring, 0),
						   KEY_NEED_SEARCH) < 0)
					continue;
			}

			/* we've got a match but we might end up racing with
			 * key_cleanup() if the keyring is currently 'dead'
			 * (ie. it has a zero usage count) */
			if (!atomic_inc_not_zero(&keyring->usage))
				continue;
			keyring->last_used_at = current_kernel_time().tv_sec;
			goto out;
		}
	}

	keyring = ERR_PTR(-ENOKEY);
out:
	read_unlock(&keyring_name_lock);
	return keyring;
}

static int keyring_detect_cycle_iterator(const void *object,
					 void *iterator_data)
{
	struct keyring_search_context *ctx = iterator_data;
	const struct key *key = keyring_ptr_to_key(object);

	kenter("{%d}", key->serial);

	/* We might get a keyring with matching index-key that is nonetheless a
	 * different keyring. */
	if (key != ctx->match_data.raw_data)
		return 0;

	ctx->result = ERR_PTR(-EDEADLK);
	return 1;
}

/*
 * See if a cycle will will be created by inserting acyclic tree B in acyclic
 * tree A at the topmost level (ie: as a direct child of A).
 *
 * Since we are adding B to A at the top level, checking for cycles should just
 * be a matter of seeing if node A is somewhere in tree B.
 */
static int keyring_detect_cycle(struct key *A, struct key *B)
{
	struct keyring_search_context ctx = {
		.index_key		= A->index_key,
		.match_data.raw_data	= A,
		.match_data.lookup_type = KEYRING_SEARCH_LOOKUP_DIRECT,
		.iterator		= keyring_detect_cycle_iterator,
		.flags			= (KEYRING_SEARCH_NO_STATE_CHECK |
					   KEYRING_SEARCH_NO_UPDATE_TIME |
					   KEYRING_SEARCH_NO_CHECK_PERM |
					   KEYRING_SEARCH_DETECT_TOO_DEEP),
	};

	rcu_read_lock();
	search_nested_keyrings(B, &ctx);
	rcu_read_unlock();
	return PTR_ERR(ctx.result) == -EAGAIN ? 0 : PTR_ERR(ctx.result);
}

/*
 * Preallocate memory so that a key can be linked into to a keyring.
 */
int __key_link_begin(struct key *keyring,
		     const struct keyring_index_key *index_key,
		     struct assoc_array_edit **_edit)
	__acquires(&keyring->sem)
	__acquires(&keyring_serialise_link_sem)
{
	struct assoc_array_edit *edit;
	int ret;

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

	BUG_ON(index_key->desc_len == 0);

	if (keyring->type != &key_type_keyring)
		return -ENOTDIR;

	down_write(&keyring->sem);

	ret = -EKEYREVOKED;
	if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
		goto error_krsem;

	/* serialise link/link calls to prevent parallel calls causing a cycle
	 * when linking two keyring in opposite orders */
	if (index_key->type == &key_type_keyring)
		down_write(&keyring_serialise_link_sem);

	/* Create an edit script that will insert/replace the key in the
	 * keyring tree.
	 */
	edit = assoc_array_insert(&keyring->keys,
				  &keyring_assoc_array_ops,
				  index_key,
				  NULL);
	if (IS_ERR(edit)) {
		ret = PTR_ERR(edit);
		goto error_sem;
	}

	/* If we're not replacing a link in-place then we're going to need some
	 * extra quota.
	 */
	if (!edit->dead_leaf) {
		ret = key_payload_reserve(keyring,
					  keyring->datalen + KEYQUOTA_LINK_BYTES);
		if (ret < 0)
			goto error_cancel;
	}

	*_edit = edit;
	kleave(" = 0");
	return 0;

error_cancel:
	assoc_array_cancel_edit(edit);
error_sem:
	if (index_key->type == &key_type_keyring)
		up_write(&keyring_serialise_link_sem);
error_krsem:
	up_write(&keyring->sem);
	kleave(" = %d", ret);
	return ret;
}

/*
 * Check already instantiated keys aren't going to be a problem.
 *
 * The caller must have called __key_link_begin(). Don't need to call this for
 * keys that were created since __key_link_begin() was called.
 */
int __key_link_check_live_key(struct key *keyring, struct key *key)
{
	if (key->type == &key_type_keyring)
		/* check that we aren't going to create a cycle by linking one
		 * keyring to another */
		return keyring_detect_cycle(keyring, key);
	return 0;
}

/*
 * Link a key into to a keyring.
 *
 * Must be called with __key_link_begin() having being called.  Discards any
 * already extant link to matching key if there is one, so that each keyring
 * holds at most one link to any given key of a particular type+description
 * combination.
 */
void __key_link(struct key *key, struct assoc_array_edit **_edit)
{
	__key_get(key);
	assoc_array_insert_set_object(*_edit, keyring_key_to_ptr(key));
	assoc_array_apply_edit(*_edit);
	*_edit = NULL;
}

/*
 * Finish linking a key into to a keyring.
 *
 * Must be called with __key_link_begin() having being called.
 */
void __key_link_end(struct key *keyring,
		    const struct keyring_index_key *index_key,
		    struct assoc_array_edit *edit)
	__releases(&keyring->sem)
	__releases(&keyring_serialise_link_sem)
{
	BUG_ON(index_key->type == NULL);
	kenter("%d,%s,", keyring->serial, index_key->type->name);

	if (index_key->type == &key_type_keyring)
		up_write(&keyring_serialise_link_sem);

	if (edit) {
		if (!edit->dead_leaf) {
			key_payload_reserve(keyring,
				keyring->datalen - KEYQUOTA_LINK_BYTES);
		}
		assoc_array_cancel_edit(edit);
	}
	up_write(&keyring->sem);
}

/*
 * Check addition of keys to restricted keyrings.
 */
static int __key_link_check_restriction(struct key *keyring, struct key *key)
{
	if (!keyring->restrict_link)
		return 0;
	return keyring->restrict_link(keyring, key->type, &key->payload);
}

/**
 * key_link - Link a key to a keyring
 * @keyring: The keyring to make the link in.
 * @key: The key to link to.
 *
 * Make a link in a keyring to a key, such that the keyring holds a reference
 * on that key and the key can potentially be found by searching that keyring.
 *
 * This function will write-lock the keyring's semaphore and will consume some
 * of the user's key data quota to hold the link.
 *
 * Returns 0 if successful, -ENOTDIR if the keyring isn't a keyring,
 * -EKEYREVOKED if the keyring has been revoked, -ENFILE if the keyring is
 * full, -EDQUOT if there is insufficient key data quota remaining to add
 * another link or -ENOMEM if there's insufficient memory.
 *
 * It is assumed that the caller has checked that it is permitted for a link to
 * be made (the keyring should have Write permission and the key Link
 * permission).
 */
int key_link(struct key *keyring, struct key *key)
{
	struct assoc_array_edit *edit;
	int ret;

	kenter("{%d,%d}", keyring->serial, atomic_read(&keyring->usage));

	key_check(keyring);
	key_check(key);

	ret = __key_link_begin(keyring, &key->index_key, &edit);
	if (ret == 0) {
		kdebug("begun {%d,%d}", keyring->serial, atomic_read(&keyring->usage));
		ret = __key_link_check_restriction(keyring, key);
		if (ret == 0)
			ret = __key_link_check_live_key(keyring, key);
		if (ret == 0)
			__key_link(key, &edit);
		__key_link_end(keyring, &key->index_key, edit);
	}

	kleave(" = %d {%d,%d}", ret, keyring->serial, atomic_read(&keyring->usage));
	return ret;
}
EXPORT_SYMBOL(key_link);

/**
 * key_unlink - Unlink the first link to a key from a keyring.
 * @keyring: The keyring to remove the link from.
 * @key: The key the link is to.
 *
 * Remove a link from a keyring to a key.
 *
 * This function will write-lock the keyring's semaphore.
 *
 * Returns 0 if successful, -ENOTDIR if the keyring isn't a keyring, -ENOENT if
 * the key isn't linked to by the keyring or -ENOMEM if there's insufficient
 * memory.
 *
 * It is assumed that the caller has checked that it is permitted for a link to
 * be removed (the keyring should have Write permission; no permissions are
 * required on the key).
 */
int key_unlink(struct key *keyring, struct key *key)
{
	struct assoc_array_edit *edit;
	int ret;

	key_check(keyring);
	key_check(key);

	if (keyring->type != &key_type_keyring)
		return -ENOTDIR;

	down_write(&keyring->sem);

	edit = assoc_array_delete(&keyring->keys, &keyring_assoc_array_ops,
				  &key->index_key);
	if (IS_ERR(edit)) {
		ret = PTR_ERR(edit);
		goto error;
	}
	ret = -ENOENT;
	if (edit == NULL)
		goto error;

	assoc_array_apply_edit(edit);
	key_payload_reserve(keyring, keyring->datalen - KEYQUOTA_LINK_BYTES);
	ret = 0;

error:
	up_write(&keyring->sem);
	return ret;
}
EXPORT_SYMBOL(key_unlink);

/**
 * keyring_clear - Clear a keyring
 * @keyring: The keyring to clear.
 *
 * Clear the contents of the specified keyring.
 *
 * Returns 0 if successful or -ENOTDIR if the keyring isn't a keyring.
 */
int keyring_clear(struct key *keyring)
{
	struct assoc_array_edit *edit;
	int ret;

	if (keyring->type != &key_type_keyring)
		return -ENOTDIR;

	down_write(&keyring->sem);

	edit = assoc_array_clear(&keyring->keys, &keyring_assoc_array_ops);
	if (IS_ERR(edit)) {
		ret = PTR_ERR(edit);
	} else {
		if (edit)
			assoc_array_apply_edit(edit);
		key_payload_reserve(keyring, 0);
		ret = 0;
	}

	up_write(&keyring->sem);
	return ret;
}
EXPORT_SYMBOL(keyring_clear);

/*
 * Dispose of the links from a revoked keyring.
 *
 * This is called with the key sem write-locked.
 */
static void keyring_revoke(struct key *keyring)
{
	struct assoc_array_edit *edit;

	edit = assoc_array_clear(&keyring->keys, &keyring_assoc_array_ops);
	if (!IS_ERR(edit)) {
		if (edit)
			assoc_array_apply_edit(edit);
		key_payload_reserve(keyring, 0);
	}
}

static bool keyring_gc_select_iterator(void *object, void *iterator_data)
{
	struct key *key = keyring_ptr_to_key(object);
	time_t *limit = iterator_data;

	if (key_is_dead(key, *limit))
		return false;
	key_get(key);
	return true;
}

static int keyring_gc_check_iterator(const void *object, void *iterator_data)
{
	const struct key *key = keyring_ptr_to_key(object);
	time_t *limit = iterator_data;

	key_check(key);
	return key_is_dead(key, *limit);
}

/*
 * Garbage collect pointers from a keyring.
 *
 * Not called with any locks held.  The keyring's key struct will not be
 * deallocated under us as only our caller may deallocate it.
 */
void keyring_gc(struct key *keyring, time_t limit)
{
	int result;

	kenter("%x{%s}", keyring->serial, keyring->description ?: "");

	if (keyring->flags & ((1 << KEY_FLAG_INVALIDATED) |
			      (1 << KEY_FLAG_REVOKED)))
		goto dont_gc;

	/* scan the keyring looking for dead keys */
	rcu_read_lock();
	result = assoc_array_iterate(&keyring->keys,
				     keyring_gc_check_iterator, &limit);
	rcu_read_unlock();
	if (result == true)
		goto do_gc;

dont_gc:
	kleave(" [no gc]");
	return;

do_gc:
	down_write(&keyring->sem);
	assoc_array_gc(&keyring->keys, &keyring_assoc_array_ops,
		       keyring_gc_select_iterator, &limit);
	up_write(&keyring->sem);
	kleave(" [gc]");
}