// SPDX-License-Identifier: GPL-2.0
/*
 * Key setup for v1 encryption policies
 *
 * Copyright 2015, 2019 Google LLC
 */

/*
 * This file implements compatibility functions for the original encryption
 * policy version ("v1"), including:
 *
 * - Deriving per-file encryption keys using the AES-128-ECB based KDF
 *   (rather than the new method of using HKDF-SHA512)
 *
 * - Retrieving fscrypt master keys from process-subscribed keyrings
 *   (rather than the new method of using a filesystem-level keyring)
 *
 * - Handling policies with the DIRECT_KEY flag set using a master key table
 *   (rather than the new method of implementing DIRECT_KEY with per-mode keys
 *    managed alongside the master keys in the filesystem-level keyring)
 */

#include <crypto/skcipher.h>
#include <crypto/utils.h>
#include <keys/user-type.h>
#include <linux/hashtable.h>
#include <linux/scatterlist.h>

#include "fscrypt_private.h"

/* Table of keys referenced by DIRECT_KEY policies */
static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */
static DEFINE_SPINLOCK(fscrypt_direct_keys_lock);

/*
 * v1 key derivation function.  This generates the derived key by encrypting the
 * master key with AES-128-ECB using the nonce as the AES key.  This provides a
 * unique derived key with sufficient entropy for each inode.  However, it's
 * nonstandard, non-extensible, doesn't evenly distribute the entropy from the
 * master key, and is trivially reversible: an attacker who compromises a
 * derived key can "decrypt" it to get back to the master key, then derive any
 * other key.  For all new code, use HKDF instead.
 *
 * The master key must be at least as long as the derived key.  If the master
 * key is longer, then only the first 'derived_keysize' bytes are used.
 */
static int derive_key_aes(const u8 *master_key,
                          const u8 nonce[FSCRYPT_FILE_NONCE_SIZE],
                          u8 *derived_key, unsigned int derived_keysize)
{
        struct crypto_sync_skcipher *tfm;
        int err;

        tfm = crypto_alloc_sync_skcipher("ecb(aes)", 0, FSCRYPT_CRYPTOAPI_MASK);
        if (IS_ERR(tfm))
                return PTR_ERR(tfm);

        err = crypto_sync_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE);
        if (err == 0) {
                SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);
                struct scatterlist src_sg, dst_sg;

                skcipher_request_set_callback(req,
                                              CRYPTO_TFM_REQ_MAY_BACKLOG |
                                                      CRYPTO_TFM_REQ_MAY_SLEEP,
                                              NULL, NULL);
                sg_init_one(&src_sg, master_key, derived_keysize);
                sg_init_one(&dst_sg, derived_key, derived_keysize);
                skcipher_request_set_crypt(req, &src_sg, &dst_sg,
                                           derived_keysize, NULL);
                err = crypto_skcipher_encrypt(req);
        }
        crypto_free_sync_skcipher(tfm);
        return err;
}

/*
 * Search the current task's subscribed keyrings for a "logon" key with
 * description prefix:descriptor, and if found acquire a read lock on it and
 * return a pointer to its validated payload in *payload_ret.
 */
static struct key *
find_and_lock_process_key(const char *prefix,
                          const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE],
                          unsigned int min_keysize,
                          const struct fscrypt_key **payload_ret)
{
        char *description;
        struct key *key;
        const struct user_key_payload *ukp;
        const struct fscrypt_key *payload;

        description = kasprintf(GFP_KERNEL, "%s%*phN", prefix,
                                FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor);
        if (!description)
                return ERR_PTR(-ENOMEM);

        key = request_key(&key_type_logon, description, NULL);
        kfree(description);
        if (IS_ERR(key))
                return key;

        down_read(&key->sem);
        ukp = user_key_payload_locked(key);

        if (!ukp) /* was the key revoked before we acquired its semaphore? */
                goto invalid;

        payload = (const struct fscrypt_key *)ukp->data;

        if (ukp->datalen != sizeof(struct fscrypt_key) ||
            payload->size < 1 || payload->size > sizeof(payload->raw)) {
                fscrypt_warn(NULL,
                             "key with description '%s' has invalid payload",
                             key->description);
                goto invalid;
        }

        if (payload->size < min_keysize) {
                fscrypt_warn(NULL,
                             "key with description '%s' is too short (got %u bytes, need %u+ bytes)",
                             key->description, payload->size, min_keysize);
                goto invalid;
        }

        *payload_ret = payload;
        return key;

invalid:
        up_read(&key->sem);
        key_put(key);
        return ERR_PTR(-ENOKEY);
}

/* Master key referenced by DIRECT_KEY policy */
struct fscrypt_direct_key {
        struct super_block              *dk_sb;
        struct hlist_node               dk_node;
        refcount_t                      dk_refcount;
        const struct fscrypt_mode       *dk_mode;
        struct fscrypt_prepared_key     dk_key;
        u8                              dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
        u8                              dk_raw[FSCRYPT_MAX_RAW_KEY_SIZE];
};

static void free_direct_key(struct fscrypt_direct_key *dk)
{
        if (dk) {
                fscrypt_destroy_prepared_key(dk->dk_sb, &dk->dk_key);
                kfree_sensitive(dk);
        }
}

void fscrypt_put_direct_key(struct fscrypt_direct_key *dk)
{
        if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock))
                return;
        hash_del(&dk->dk_node);
        spin_unlock(&fscrypt_direct_keys_lock);

        free_direct_key(dk);
}

/*
 * Find/insert the given key into the fscrypt_direct_keys table.  If found, it
 * is returned with elevated refcount, and 'to_insert' is freed if non-NULL.  If
 * not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise
 * NULL is returned.
 */
static struct fscrypt_direct_key *
find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
                          const u8 *raw_key,
                          const struct fscrypt_inode_info *ci)
{
        unsigned long hash_key;
        struct fscrypt_direct_key *dk;

        /*
         * Careful: to avoid potentially leaking secret key bytes via timing
         * information, we must key the hash table by descriptor rather than by
         * raw key, and use crypto_memneq() when comparing raw keys.
         */

        BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE);
        memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor,
               sizeof(hash_key));

        spin_lock(&fscrypt_direct_keys_lock);
        hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) {
                if (memcmp(ci->ci_policy.v1.master_key_descriptor,
                           dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
                        continue;
                if (ci->ci_mode != dk->dk_mode)
                        continue;
                if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
                        continue;
                if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
                        continue;
                /* using existing tfm with same (descriptor, mode, raw_key) */
                refcount_inc(&dk->dk_refcount);
                spin_unlock(&fscrypt_direct_keys_lock);
                free_direct_key(to_insert);
                return dk;
        }
        if (to_insert)
                hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key);
        spin_unlock(&fscrypt_direct_keys_lock);
        return to_insert;
}

/* Prepare to encrypt directly using the master key in the given mode */
static struct fscrypt_direct_key *
fscrypt_get_direct_key(const struct fscrypt_inode_info *ci, const u8 *raw_key)
{
        struct fscrypt_direct_key *dk;
        int err;

        /* Is there already a tfm for this key? */
        dk = find_or_insert_direct_key(NULL, raw_key, ci);
        if (dk)
                return dk;

        /* Nope, allocate one. */
        dk = kzalloc_obj(*dk);
        if (!dk)
                return ERR_PTR(-ENOMEM);
        dk->dk_sb = ci->ci_inode->i_sb;
        refcount_set(&dk->dk_refcount, 1);
        dk->dk_mode = ci->ci_mode;
        err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci);
        if (err)
                goto err_free_dk;
        memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor,
               FSCRYPT_KEY_DESCRIPTOR_SIZE);
        memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize);

        return find_or_insert_direct_key(dk, raw_key, ci);

err_free_dk:
        free_direct_key(dk);
        return ERR_PTR(err);
}

/* v1 policy, DIRECT_KEY: use the master key directly */
static int setup_v1_file_key_direct(struct fscrypt_inode_info *ci,
                                    const u8 *raw_master_key)
{
        struct fscrypt_direct_key *dk;

        dk = fscrypt_get_direct_key(ci, raw_master_key);
        if (IS_ERR(dk))
                return PTR_ERR(dk);
        ci->ci_direct_key = dk;
        ci->ci_enc_key = dk->dk_key;
        return 0;
}

/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
                                     const u8 *raw_master_key)
{
        u8 *derived_key;
        int err;

        /*
         * This cannot be a stack buffer because it will be passed to the
         * scatterlist crypto API during derive_key_aes().
         */
        derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
        if (!derived_key)
                return -ENOMEM;

        err = derive_key_aes(raw_master_key, ci->ci_nonce,
                             derived_key, ci->ci_mode->keysize);
        if (err)
                goto out;

        err = fscrypt_set_per_file_enc_key(ci, derived_key);
out:
        kfree_sensitive(derived_key);
        return err;
}

int fscrypt_setup_v1_file_key(struct fscrypt_inode_info *ci,
                              const u8 *raw_master_key)
{
        if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY)
                return setup_v1_file_key_direct(ci, raw_master_key);
        else
                return setup_v1_file_key_derived(ci, raw_master_key);
}

int
fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_inode_info *ci)
{
        const struct super_block *sb = ci->ci_inode->i_sb;
        struct key *key;
        const struct fscrypt_key *payload;
        int err;

        key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
                                        ci->ci_policy.v1.master_key_descriptor,
                                        ci->ci_mode->keysize, &payload);
        if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) {
                key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix,
                                                ci->ci_policy.v1.master_key_descriptor,
                                                ci->ci_mode->keysize, &payload);
        }
        if (IS_ERR(key))
                return PTR_ERR(key);

        err = fscrypt_setup_v1_file_key(ci, payload->raw);
        up_read(&key->sem);
        key_put(key);
        return err;
}