libs/devicemapper/src/defaultkey.rs

/*
 * Copyright (C) 2025 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/// `default-key` module implements the "default-key" target in the device mapper
/// framework. Specifically, it provides `DmDefaultKeyTargetBuilder` struct which
/// is used to construct a `DmDefaultKeyTarget` struct which is then given to
/// `DeviceMapper` to create a mapper device.
use crate::crypt::CipherType;
use crate::DmTargetSpec;

use anyhow::{ensure, Context, Result};
use std::io::Write;
use std::mem::size_of;
use std::path::Path;
use zerocopy::IntoBytes;

const SECTOR_SIZE: u64 = 512;

pub struct DmDefaultKeyTarget(Box<[u8]>);

/// Device-Mapper’s dm-default-key target is designed to be used in conjunction with
/// file based encryption in either ext4 or f2fs and provides encryption via a default
/// key of blocks, typically metadata blocks, that the file system does not encrypt.
impl DmDefaultKeyTarget {
    /// Flatten into slice
    pub fn as_slice(&self) -> &[u8] {
        self.0.as_ref()
    }
}

pub struct DmDefaultKeyTargetBuilder<'a> {
    cipher: CipherType,
    key: Option<&'a [u8]>,
    iv_offset: u64,
    device_path: Option<&'a Path>,
    offset: u64,
    device_size: u64,
}

/// A builder that constructs `DmDefaultKeyTarget` struct.
impl Default for DmDefaultKeyTargetBuilder<'_> {
    fn default() -> Self {
        DmDefaultKeyTargetBuilder {
            cipher: CipherType::AES256XTS,
            key: None,
            iv_offset: 0,
            device_path: None,
            offset: 0,
            device_size: 0,
        }
    }
}

impl<'a> DmDefaultKeyTargetBuilder<'a> {
    /// Sets the device that will be used as the data device (i.e. providing actual data).
    pub fn data_device(&mut self, p: &'a Path, size: u64) -> &mut Self {
        self.device_path = Some(p);
        self.device_size = size;
        self
    }

    /// Sets the encryption cipher.
    pub fn cipher(&mut self, cipher: CipherType) -> &mut Self {
        self.cipher = cipher;
        self
    }

    /// Sets the key used for encryption. Input is byte array.
    pub fn key(&mut self, key: &'a [u8]) -> &mut Self {
        self.key = Some(key);
        self
    }

    /// The IV offset is a sector count that is added to the sector number before creating the IV.
    pub fn iv_offset(&mut self, iv_offset: u64) -> &mut Self {
        self.iv_offset = iv_offset;
        self
    }

    /// Starting sector within the device where the encrypted data begins
    pub fn offset(&mut self, offset: u64) -> &mut Self {
        self.offset = offset;
        self
    }

    /// Constructs a `DmDefaultKeyTarget`.
    pub fn build(&self) -> Result<DmDefaultKeyTarget> {
        // The `DmDefaultKeyTarget` struct actually is a flattened data consisting of a header and
        // body. The format of the header is `dm_target_spec` as defined in
        // include/uapi/linux/dm-ioctl.h.
        let device_path = self
            .device_path
            .context("data device is not set")?
            .to_str()
            .context("data device path is not encoded in utf8")?;

        ensure!(self.key.is_some(), "key is not set");
        // Unwrap is safe because we already made sure key.is_some()
        ensure!(
            self.cipher.validata_key_size(self.key.unwrap().len()),
            format!("Invalid key size for cipher:{}", self.cipher.get_kernel_crypto_name())
        );
        let key = hex::encode(self.key.unwrap());

        // Step2: serialize the information according to the spec, which is ...
        // DmTargetSpec{...}
        // <cipher> <key> <iv_offset> <device path> <offset>
        let mut body = String::new();
        use std::fmt::Write;
        write!(&mut body, "{} ", self.cipher.get_kernel_crypto_name())?;
        write!(&mut body, "{key} ")?;
        write!(&mut body, "{} ", self.iv_offset)?;
        write!(&mut body, "{device_path} ")?;
        write!(&mut body, "{} ", self.offset)?;
        write!(&mut body, "\0")?; // null terminator

        let size = size_of::<DmTargetSpec>() + body.len();
        let aligned_size = (size + 7) & !7; // align to 8 byte boundaries
        let padding = aligned_size - size;

        let mut header = DmTargetSpec::new("default-key")?;
        header.sector_start = 0;
        header.length = self.device_size / SECTOR_SIZE; // number of 512-byte sectors
        header.next = aligned_size as u32;

        let mut buf = Vec::with_capacity(aligned_size);
        buf.write_all(header.as_bytes())?;
        buf.write_all(body.as_bytes())?;
        buf.write_all(vec![0; padding].as_slice())?;
        Ok(DmDefaultKeyTarget(buf.into_boxed_slice()))
    }
}