static int validate_user_key(struct fscrypt_info *crypt_info,
			struct fscrypt_context *ctx, u8 *raw_key,
			const char *prefix)
{
	char *description;
	struct key *keyring_key;
	struct fscrypt_key *master_key;
	const struct user_key_payload *ukp;
	int res;

	description = kasprintf(GFP_NOFS, "%s%*phN", prefix,
				FS_KEY_DESCRIPTOR_SIZE,
				ctx->master_key_descriptor);
	if (!description)
		return -ENOMEM;

	keyring_key = request_key(&key_type_logon, description, NULL);
	kfree(description);
	if (IS_ERR(keyring_key))
		return PTR_ERR(keyring_key);
	down_read(&keyring_key->sem);

	if (keyring_key->type != &key_type_logon) {
		printk_once(KERN_WARNING
				"%s: key type must be logon\n", __func__);
		res = -ENOKEY;
		goto out;
	}
	ukp = user_key_payload(keyring_key);
	if (ukp->datalen != sizeof(struct fscrypt_key)) {
		res = -EINVAL;
		goto out;
	}
	master_key = (struct fscrypt_key *)ukp->data;
	BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);

	if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
		printk_once(KERN_WARNING
				"%s: key size incorrect: %d\n",
				__func__, master_key->size);
		res = -ENOKEY;
		goto out;
	}
	res = derive_key_aes(ctx->nonce, master_key->raw, raw_key);
out:
	up_read(&keyring_key->sem);
	key_put(keyring_key);
	return res;
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.