text: // SPDX-License-Identifier: GPL-2.0-or-later
  /* Large capacity key type
   *
   * Copyright (C) 2017-2020 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
   * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
   * Written by David Howells (dhowells@redhat.com)
   */
  
  #define pr_fmt(fmt) "big_key: "fmt
  #include <linux/init.h>
  #include <linux/seq_file.h>
  #include <linux/file.h>
  #include <linux/shmem_fs.h>
  #include <linux/err.h>
  #include <linux/random.h>
  #include <keys/user-type.h>
  #include <keys/big_key-type.h>
  #include <crypto/chacha20poly1305.h>
  
  /*
   * Layout of key payload words.
   */
  enum {
  	big_key_data,
  	big_key_path,
  	big_key_path_2nd_part,
  	big_key_len,
  };
  
  /*
   * If the data is under this limit, there's no point creating a shm file to
   * hold it as the permanently resident metadata for the shmem fs will be at
   * least as large as the data.
   */
  #define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))
  
  /*
   * big_key defined keys take an arbitrary string as the description and an
   * arbitrary blob of data as the payload
   */
  struct key_type key_type_big_key = {
  	.name			= "big_key",
  	.preparse		= big_key_preparse,
  	.free_preparse		= big_key_free_preparse,
  	.instantiate		= generic_key_instantiate,
  	.revoke			= big_key_revoke,
  	.destroy		= big_key_destroy,
  	.describe		= big_key_describe,
  	.read			= big_key_read,
  	.update			= big_key_update,
  };
  
  /*
   * Preparse a big key
   */
  int big_key_preparse(struct key_preparsed_payload *prep)
  {
  	struct path *path = (struct path *)&prep->payload.data[big_key_path];
  	struct file *file;
  	u8 *buf, *enckey;
  	ssize_t written;
  	size_t datalen = prep->datalen;
  	size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
  	int ret;
  
  	if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
  		return -EINVAL;
  
  	/* Set an arbitrary quota */
  	prep->quotalen = 16;
  
  	prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;
  
  	if (datalen > BIG_KEY_FILE_THRESHOLD) {
  		/* Create a shmem file to store the data in.  This will permit the data
  		 * to be swapped out if needed.
  		 *
  		 * File content is stored encrypted with randomly generated key.
  		 * Since the key is random for each file, we can set the nonce
  		 * to zero, provided we never define a ->update() call.
  		 */
  		loff_t pos = 0;
  
  		buf = kvmalloc(enclen, GFP_KERNEL);
  		if (!buf)
  			return -ENOMEM;
  
  		/* generate random key */
  		enckey = kmalloc(CHACHA20POLY1305_KEY_SIZE, GFP_KERNEL);
  		if (!enckey) {
  			ret = -ENOMEM;
  			goto error;
  		}
  		ret = get_random_bytes_wait(enckey, CHACHA20POLY1305_KEY_SIZE);
  		if (unlikely(ret))
  			goto err_enckey;
  
  		/* encrypt data */
  		chacha20poly1305_encrypt(buf, prep->data, datalen, NULL, 0,
  					 0, enckey);
  
  		/* save aligned data to file */
  		file = shmem_kernel_file_setup("", enclen, 0);
  		if (IS_ERR(file)) {
  			ret = PTR_ERR(file);
  			goto err_enckey;
  		}
  
  		written = kernel_write(file, buf, enclen, &pos);
  		if (written != enclen) {
  			ret = written;
  			if (written >= 0)
  				ret = -EIO;
  			goto err_fput;
  		}
  
  		/* Pin the mount and dentry to the key so that we can open it again
  		 * later
  		 */
  		prep->payload.data[big_key_data] = enckey;
  		*path = file->f_path;
  		path_get(path);
  		fput(file);
  		kvfree_sensitive(buf, enclen);
  	} else {
  		/* Just store the data in a buffer */
  		void *data = kmalloc(datalen, GFP_KERNEL);
  
  		if (!data)
  			return -ENOMEM;
  
  		prep->payload.data[big_key_data] = data;
  		memcpy(data, prep->data, prep->datalen);
  	}
  	return 0;
  
  err_fput:
  	fput(file);
  err_enckey:
  	kfree_sensitive(enckey);
  error:
  	kvfree_sensitive(buf, enclen);
  	return ret;
  }
  
  /*
   * Clear preparsement.
   */
  void big_key_free_preparse(struct key_preparsed_payload *prep)
  {
  	if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
  		struct path *path = (struct path *)&prep->payload.data[big_key_path];
  
  		path_put(path);
  	}
  	kfree_sensitive(prep->payload.data[big_key_data]);
  }
  
  /*
   * dispose of the links from a revoked keyring
   * - called with the key sem write-locked
   */
  void big_key_revoke(struct key *key)
  {
  	struct path *path = (struct path *)&key->payload.data[big_key_path];
  
  	/* clear the quota */
  	key_payload_reserve(key, 0);
  	if (key_is_positive(key) &&
  	    (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
  		vfs_truncate(path, 0);
  }
  
  /*
   * dispose of the data dangling from the corpse of a big_key key
   */
  void big_key_destroy(struct key *key)
  {
  	size_t datalen = (size_t)key->payload.data[big_key_len];
  
  	if (datalen > BIG_KEY_FILE_THRESHOLD) {
  		struct path *path = (struct path *)&key->payload.data[big_key_path];
  
  		path_put(path);
  		path->mnt = NULL;
  		path->dentry = NULL;
  	}
  	kfree_sensitive(key->payload.data[big_key_data]);
  	key->payload.data[big_key_data] = NULL;
  }
  
  /*
   * Update a big key
   */
  int big_key_update(struct key *key, struct key_preparsed_payload *prep)
  {
  	int ret;
  
  	ret = key_payload_reserve(key, prep->datalen);
  	if (ret < 0)
  		return ret;
  
  	if (key_is_positive(key))
  		big_key_destroy(key);
  
  	return generic_key_instantiate(key, prep);
  }
  
  /*
   * describe the big_key key
   */
  void big_key_describe(const struct key *key, struct seq_file *m)
  {
  	size_t datalen = (size_t)key->payload.data[big_key_len];
  
  	seq_puts(m, key->description);
  
  	if (key_is_positive(key))
  		seq_printf(m, ": %zu [%s]",
  			   datalen,
  			   datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
  }
  
  /*
   * read the key data
   * - the key's semaphore is read-locked
   */
  long big_key_read(const struct key *key, char *buffer, size_t buflen)
  {
  	size_t datalen = (size_t)key->payload.data[big_key_len];
  	long ret;
  
  	if (!buffer || buflen < datalen)
  		return datalen;
  
  	if (datalen > BIG_KEY_FILE_THRESHOLD) {
  		struct path *path = (struct path *)&key->payload.data[big_key_path];
  		struct file *file;
  		u8 *buf, *enckey = (u8 *)key->payload.data[big_key_data];
  		size_t enclen = datalen + CHACHA20POLY1305_AUTHTAG_SIZE;
  		loff_t pos = 0;
  
  		buf = kvmalloc(enclen, GFP_KERNEL);
  		if (!buf)
  			return -ENOMEM;
  
  		file = dentry_open(path, O_RDONLY, current_cred());
  		if (IS_ERR(file)) {
  			ret = PTR_ERR(file);
  			goto error;
  		}
  
  		/* read file to kernel and decrypt */
  		ret = kernel_read(file, buf, enclen, &pos);
  		if (ret != enclen) {
  			if (ret >= 0)
  				ret = -EIO;
  			goto err_fput;
  		}
  
  		ret = chacha20poly1305_decrypt(buf, buf, enclen, NULL, 0, 0,
  					       enckey) ? 0 : -EBADMSG;
  		if (unlikely(ret))
  			goto err_fput;
  
  		ret = datalen;
  
  		/* copy out decrypted data */
  		memcpy(buffer, buf, datalen);
  
  err_fput:
  		fput(file);
  error:
  		kvfree_sensitive(buf, enclen);
  	} else {
  		ret = datalen;
  		memcpy(buffer, key->payload.data[big_key_data], datalen);
  	}
  
  	return ret;
  }
  
  /*
   * Register key type
   */
  static int __init big_key_init(void)
  {
  	return register_key_type(&key_type_big_key);
  }
  
  late_initcall(big_key_init);
  