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319 lines
9.3 KiB
319 lines
9.3 KiB
// SPDX-License-Identifier: GPL-2.0 |
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/* |
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* Key setup for v1 encryption policies |
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* |
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* Copyright 2015, 2019 Google LLC |
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*/ |
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|
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/* |
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* This file implements compatibility functions for the original encryption |
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* policy version ("v1"), including: |
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* |
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* - Deriving per-file encryption keys using the AES-128-ECB based KDF |
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* (rather than the new method of using HKDF-SHA512) |
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* |
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* - Retrieving fscrypt master keys from process-subscribed keyrings |
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* (rather than the new method of using a filesystem-level keyring) |
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* |
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* - Handling policies with the DIRECT_KEY flag set using a master key table |
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* (rather than the new method of implementing DIRECT_KEY with per-mode keys |
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* managed alongside the master keys in the filesystem-level keyring) |
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*/ |
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#include <crypto/algapi.h> |
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#include <crypto/skcipher.h> |
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#include <keys/user-type.h> |
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#include <linux/hashtable.h> |
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#include <linux/scatterlist.h> |
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#include "fscrypt_private.h" |
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/* Table of keys referenced by DIRECT_KEY policies */ |
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static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */ |
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static DEFINE_SPINLOCK(fscrypt_direct_keys_lock); |
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/* |
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* v1 key derivation function. This generates the derived key by encrypting the |
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* master key with AES-128-ECB using the nonce as the AES key. This provides a |
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* unique derived key with sufficient entropy for each inode. However, it's |
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* nonstandard, non-extensible, doesn't evenly distribute the entropy from the |
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* master key, and is trivially reversible: an attacker who compromises a |
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* derived key can "decrypt" it to get back to the master key, then derive any |
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* other key. For all new code, use HKDF instead. |
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* |
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* The master key must be at least as long as the derived key. If the master |
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* key is longer, then only the first 'derived_keysize' bytes are used. |
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*/ |
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static int derive_key_aes(const u8 *master_key, |
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const u8 nonce[FSCRYPT_FILE_NONCE_SIZE], |
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u8 *derived_key, unsigned int derived_keysize) |
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{ |
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int res = 0; |
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struct skcipher_request *req = NULL; |
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DECLARE_CRYPTO_WAIT(wait); |
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struct scatterlist src_sg, dst_sg; |
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struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0); |
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if (IS_ERR(tfm)) { |
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res = PTR_ERR(tfm); |
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tfm = NULL; |
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goto out; |
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} |
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crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS); |
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req = skcipher_request_alloc(tfm, GFP_KERNEL); |
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if (!req) { |
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res = -ENOMEM; |
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goto out; |
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} |
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skcipher_request_set_callback(req, |
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CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP, |
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crypto_req_done, &wait); |
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res = crypto_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE); |
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if (res < 0) |
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goto out; |
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sg_init_one(&src_sg, master_key, derived_keysize); |
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sg_init_one(&dst_sg, derived_key, derived_keysize); |
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skcipher_request_set_crypt(req, &src_sg, &dst_sg, derived_keysize, |
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NULL); |
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res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait); |
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out: |
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skcipher_request_free(req); |
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crypto_free_skcipher(tfm); |
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return res; |
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} |
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/* |
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* Search the current task's subscribed keyrings for a "logon" key with |
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* description prefix:descriptor, and if found acquire a read lock on it and |
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* return a pointer to its validated payload in *payload_ret. |
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*/ |
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static struct key * |
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find_and_lock_process_key(const char *prefix, |
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const u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE], |
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unsigned int min_keysize, |
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const struct fscrypt_key **payload_ret) |
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{ |
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char *description; |
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struct key *key; |
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const struct user_key_payload *ukp; |
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const struct fscrypt_key *payload; |
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description = kasprintf(GFP_KERNEL, "%s%*phN", prefix, |
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FSCRYPT_KEY_DESCRIPTOR_SIZE, descriptor); |
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if (!description) |
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return ERR_PTR(-ENOMEM); |
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key = request_key(&key_type_logon, description, NULL); |
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kfree(description); |
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if (IS_ERR(key)) |
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return key; |
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down_read(&key->sem); |
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ukp = user_key_payload_locked(key); |
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if (!ukp) /* was the key revoked before we acquired its semaphore? */ |
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goto invalid; |
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payload = (const struct fscrypt_key *)ukp->data; |
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if (ukp->datalen != sizeof(struct fscrypt_key) || |
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payload->size < 1 || payload->size > FSCRYPT_MAX_KEY_SIZE) { |
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fscrypt_warn(NULL, |
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"key with description '%s' has invalid payload", |
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key->description); |
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goto invalid; |
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} |
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if (payload->size < min_keysize) { |
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fscrypt_warn(NULL, |
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"key with description '%s' is too short (got %u bytes, need %u+ bytes)", |
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key->description, payload->size, min_keysize); |
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goto invalid; |
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} |
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*payload_ret = payload; |
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return key; |
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invalid: |
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up_read(&key->sem); |
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key_put(key); |
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return ERR_PTR(-ENOKEY); |
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} |
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/* Master key referenced by DIRECT_KEY policy */ |
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struct fscrypt_direct_key { |
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struct hlist_node dk_node; |
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refcount_t dk_refcount; |
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const struct fscrypt_mode *dk_mode; |
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struct fscrypt_prepared_key dk_key; |
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u8 dk_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; |
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u8 dk_raw[FSCRYPT_MAX_KEY_SIZE]; |
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}; |
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static void free_direct_key(struct fscrypt_direct_key *dk) |
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{ |
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if (dk) { |
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fscrypt_destroy_prepared_key(&dk->dk_key); |
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kfree_sensitive(dk); |
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} |
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} |
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void fscrypt_put_direct_key(struct fscrypt_direct_key *dk) |
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{ |
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if (!refcount_dec_and_lock(&dk->dk_refcount, &fscrypt_direct_keys_lock)) |
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return; |
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hash_del(&dk->dk_node); |
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spin_unlock(&fscrypt_direct_keys_lock); |
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free_direct_key(dk); |
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} |
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/* |
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* Find/insert the given key into the fscrypt_direct_keys table. If found, it |
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* is returned with elevated refcount, and 'to_insert' is freed if non-NULL. If |
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* not found, 'to_insert' is inserted and returned if it's non-NULL; otherwise |
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* NULL is returned. |
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*/ |
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static struct fscrypt_direct_key * |
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find_or_insert_direct_key(struct fscrypt_direct_key *to_insert, |
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const u8 *raw_key, const struct fscrypt_info *ci) |
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{ |
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unsigned long hash_key; |
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struct fscrypt_direct_key *dk; |
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/* |
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* Careful: to avoid potentially leaking secret key bytes via timing |
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* information, we must key the hash table by descriptor rather than by |
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* raw key, and use crypto_memneq() when comparing raw keys. |
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*/ |
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BUILD_BUG_ON(sizeof(hash_key) > FSCRYPT_KEY_DESCRIPTOR_SIZE); |
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memcpy(&hash_key, ci->ci_policy.v1.master_key_descriptor, |
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sizeof(hash_key)); |
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spin_lock(&fscrypt_direct_keys_lock); |
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hash_for_each_possible(fscrypt_direct_keys, dk, dk_node, hash_key) { |
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if (memcmp(ci->ci_policy.v1.master_key_descriptor, |
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dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0) |
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continue; |
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if (ci->ci_mode != dk->dk_mode) |
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continue; |
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if (!fscrypt_is_key_prepared(&dk->dk_key, ci)) |
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continue; |
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if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize)) |
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continue; |
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/* using existing tfm with same (descriptor, mode, raw_key) */ |
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refcount_inc(&dk->dk_refcount); |
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spin_unlock(&fscrypt_direct_keys_lock); |
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free_direct_key(to_insert); |
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return dk; |
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} |
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if (to_insert) |
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hash_add(fscrypt_direct_keys, &to_insert->dk_node, hash_key); |
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spin_unlock(&fscrypt_direct_keys_lock); |
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return to_insert; |
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} |
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/* Prepare to encrypt directly using the master key in the given mode */ |
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static struct fscrypt_direct_key * |
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fscrypt_get_direct_key(const struct fscrypt_info *ci, const u8 *raw_key) |
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{ |
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struct fscrypt_direct_key *dk; |
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int err; |
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/* Is there already a tfm for this key? */ |
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dk = find_or_insert_direct_key(NULL, raw_key, ci); |
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if (dk) |
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return dk; |
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/* Nope, allocate one. */ |
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dk = kzalloc(sizeof(*dk), GFP_KERNEL); |
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if (!dk) |
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return ERR_PTR(-ENOMEM); |
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refcount_set(&dk->dk_refcount, 1); |
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dk->dk_mode = ci->ci_mode; |
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err = fscrypt_prepare_key(&dk->dk_key, raw_key, ci); |
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if (err) |
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goto err_free_dk; |
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memcpy(dk->dk_descriptor, ci->ci_policy.v1.master_key_descriptor, |
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FSCRYPT_KEY_DESCRIPTOR_SIZE); |
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memcpy(dk->dk_raw, raw_key, ci->ci_mode->keysize); |
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return find_or_insert_direct_key(dk, raw_key, ci); |
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err_free_dk: |
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free_direct_key(dk); |
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return ERR_PTR(err); |
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} |
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/* v1 policy, DIRECT_KEY: use the master key directly */ |
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static int setup_v1_file_key_direct(struct fscrypt_info *ci, |
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const u8 *raw_master_key) |
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{ |
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struct fscrypt_direct_key *dk; |
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dk = fscrypt_get_direct_key(ci, raw_master_key); |
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if (IS_ERR(dk)) |
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return PTR_ERR(dk); |
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ci->ci_direct_key = dk; |
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ci->ci_enc_key = dk->dk_key; |
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return 0; |
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} |
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/* v1 policy, !DIRECT_KEY: derive the file's encryption key */ |
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static int setup_v1_file_key_derived(struct fscrypt_info *ci, |
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const u8 *raw_master_key) |
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{ |
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u8 *derived_key; |
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int err; |
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/* |
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* This cannot be a stack buffer because it will be passed to the |
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* scatterlist crypto API during derive_key_aes(). |
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*/ |
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derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL); |
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if (!derived_key) |
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return -ENOMEM; |
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err = derive_key_aes(raw_master_key, ci->ci_nonce, |
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derived_key, ci->ci_mode->keysize); |
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if (err) |
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goto out; |
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err = fscrypt_set_per_file_enc_key(ci, derived_key); |
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out: |
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kfree_sensitive(derived_key); |
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return err; |
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} |
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int fscrypt_setup_v1_file_key(struct fscrypt_info *ci, const u8 *raw_master_key) |
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{ |
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if (ci->ci_policy.v1.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) |
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return setup_v1_file_key_direct(ci, raw_master_key); |
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else |
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return setup_v1_file_key_derived(ci, raw_master_key); |
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} |
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int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci) |
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{ |
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struct key *key; |
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const struct fscrypt_key *payload; |
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int err; |
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key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX, |
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ci->ci_policy.v1.master_key_descriptor, |
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ci->ci_mode->keysize, &payload); |
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if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) { |
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key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix, |
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ci->ci_policy.v1.master_key_descriptor, |
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ci->ci_mode->keysize, &payload); |
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} |
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if (IS_ERR(key)) |
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return PTR_ERR(key); |
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err = fscrypt_setup_v1_file_key(ci, payload->raw); |
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up_read(&key->sem); |
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key_put(key); |
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return err; |
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}
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