pvmfw/avb/src/verify.rs

// Copyright 2022, 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.

//! This module handles the pvmfw payload verification.

use crate::error::{slot_verify_result_to_verify_payload_result, AvbSlotVerifyError};
use avb_bindgen::{avb_slot_verify, AvbHashtreeErrorMode, AvbIOResult, AvbOps, AvbSlotVerifyFlags};
use core::{
    ffi::{c_char, c_void, CStr},
    ptr::{self, NonNull},
    slice,
};

static NULL_BYTE: &[u8] = b"\0";

enum AvbIOError {
    /// AVB_IO_RESULT_ERROR_OOM,
    #[allow(dead_code)]
    Oom,
    /// AVB_IO_RESULT_ERROR_IO,
    #[allow(dead_code)]
    Io,
    /// AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION,
    NoSuchPartition,
    /// AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION,
    RangeOutsidePartition,
    /// AVB_IO_RESULT_ERROR_NO_SUCH_VALUE,
    NoSuchValue,
    /// AVB_IO_RESULT_ERROR_INVALID_VALUE_SIZE,
    InvalidValueSize,
    /// AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE,
    #[allow(dead_code)]
    InsufficientSpace,
}

impl From<AvbIOError> for AvbIOResult {
    fn from(error: AvbIOError) -> Self {
        match error {
            AvbIOError::Oom => AvbIOResult::AVB_IO_RESULT_ERROR_OOM,
            AvbIOError::Io => AvbIOResult::AVB_IO_RESULT_ERROR_IO,
            AvbIOError::NoSuchPartition => AvbIOResult::AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION,
            AvbIOError::RangeOutsidePartition => {
                AvbIOResult::AVB_IO_RESULT_ERROR_RANGE_OUTSIDE_PARTITION
            }
            AvbIOError::NoSuchValue => AvbIOResult::AVB_IO_RESULT_ERROR_NO_SUCH_VALUE,
            AvbIOError::InvalidValueSize => AvbIOResult::AVB_IO_RESULT_ERROR_INVALID_VALUE_SIZE,
            AvbIOError::InsufficientSpace => AvbIOResult::AVB_IO_RESULT_ERROR_INSUFFICIENT_SPACE,
        }
    }
}

fn to_avb_io_result(result: Result<(), AvbIOError>) -> AvbIOResult {
    result.map_or_else(|e| e.into(), |_| AvbIOResult::AVB_IO_RESULT_OK)
}

extern "C" fn read_is_device_unlocked(
    _ops: *mut AvbOps,
    out_is_unlocked: *mut bool,
) -> AvbIOResult {
    to_avb_io_result(write(out_is_unlocked, false))
}

unsafe extern "C" fn get_preloaded_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    num_bytes: usize,
    out_pointer: *mut *mut u8,
    out_num_bytes_preloaded: *mut usize,
) -> AvbIOResult {
    to_avb_io_result(try_get_preloaded_partition(
        ops,
        partition,
        num_bytes,
        out_pointer,
        out_num_bytes_preloaded,
    ))
}

fn try_get_preloaded_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    num_bytes: usize,
    out_pointer: *mut *mut u8,
    out_num_bytes_preloaded: *mut usize,
) -> Result<(), AvbIOError> {
    let ops = as_ref(ops)?;
    let partition = ops.as_ref().get_partition(partition)?;
    write(out_pointer, partition.as_ptr() as *mut u8)?;
    write(out_num_bytes_preloaded, partition.len().min(num_bytes))
}

extern "C" fn read_from_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    offset: i64,
    num_bytes: usize,
    buffer: *mut c_void,
    out_num_read: *mut usize,
) -> AvbIOResult {
    to_avb_io_result(try_read_from_partition(
        ops,
        partition,
        offset,
        num_bytes,
        buffer,
        out_num_read,
    ))
}

fn try_read_from_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    offset: i64,
    num_bytes: usize,
    buffer: *mut c_void,
    out_num_read: *mut usize,
) -> Result<(), AvbIOError> {
    let ops = as_ref(ops)?;
    let partition = ops.as_ref().get_partition(partition)?;
    let buffer = to_nonnull(buffer)?;
    // SAFETY: It is safe to copy the requested number of bytes to `buffer` as `buffer`
    // is created to point to the `num_bytes` of bytes in memory.
    let buffer_slice = unsafe { slice::from_raw_parts_mut(buffer.as_ptr() as *mut u8, num_bytes) };
    copy_data_to_dst(partition, offset, buffer_slice)?;
    write(out_num_read, buffer_slice.len())
}

fn copy_data_to_dst(src: &[u8], offset: i64, dst: &mut [u8]) -> Result<(), AvbIOError> {
    let start = to_copy_start(offset, src.len()).ok_or(AvbIOError::InvalidValueSize)?;
    let end = start.checked_add(dst.len()).ok_or(AvbIOError::InvalidValueSize)?;
    dst.copy_from_slice(src.get(start..end).ok_or(AvbIOError::RangeOutsidePartition)?);
    Ok(())
}

fn to_copy_start(offset: i64, len: usize) -> Option<usize> {
    usize::try_from(offset)
        .ok()
        .or_else(|| isize::try_from(offset).ok().and_then(|v| len.checked_add_signed(v)))
}

extern "C" fn get_size_of_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    out_size_num_bytes: *mut u64,
) -> AvbIOResult {
    to_avb_io_result(try_get_size_of_partition(ops, partition, out_size_num_bytes))
}

fn try_get_size_of_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    out_size_num_bytes: *mut u64,
) -> Result<(), AvbIOError> {
    let ops = as_ref(ops)?;
    let partition = ops.as_ref().get_partition(partition)?;
    let partition_size =
        u64::try_from(partition.len()).map_err(|_| AvbIOError::InvalidValueSize)?;
    write(out_size_num_bytes, partition_size)
}

extern "C" fn read_rollback_index(
    _ops: *mut AvbOps,
    _rollback_index_location: usize,
    _out_rollback_index: *mut u64,
) -> AvbIOResult {
    // Rollback protection is not yet implemented, but
    // this method is required by `avb_slot_verify()`.
    AvbIOResult::AVB_IO_RESULT_OK
}

extern "C" fn get_unique_guid_for_partition(
    _ops: *mut AvbOps,
    _partition: *const c_char,
    _guid_buf: *mut c_char,
    _guid_buf_size: usize,
) -> AvbIOResult {
    // This method is required by `avb_slot_verify()`.
    AvbIOResult::AVB_IO_RESULT_OK
}

extern "C" fn validate_public_key_for_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    public_key_data: *const u8,
    public_key_length: usize,
    public_key_metadata: *const u8,
    public_key_metadata_length: usize,
    out_is_trusted: *mut bool,
    out_rollback_index_location: *mut u32,
) -> AvbIOResult {
    to_avb_io_result(try_validate_public_key_for_partition(
        ops,
        partition,
        public_key_data,
        public_key_length,
        public_key_metadata,
        public_key_metadata_length,
        out_is_trusted,
        out_rollback_index_location,
    ))
}

#[allow(clippy::too_many_arguments)]
fn try_validate_public_key_for_partition(
    ops: *mut AvbOps,
    partition: *const c_char,
    public_key_data: *const u8,
    public_key_length: usize,
    _public_key_metadata: *const u8,
    _public_key_metadata_length: usize,
    out_is_trusted: *mut bool,
    _out_rollback_index_location: *mut u32,
) -> Result<(), AvbIOError> {
    is_not_null(public_key_data)?;
    // SAFETY: It is safe to create a slice with the given pointer and length as
    // `public_key_data` is a valid pointer and it points to an array of length
    // `public_key_length`.
    let public_key = unsafe { slice::from_raw_parts(public_key_data, public_key_length) };
    let ops = as_ref(ops)?;
    // Verifies the public key for the known partitions only.
    ops.as_ref().get_partition(partition)?;
    let trusted_public_key = ops.as_ref().trusted_public_key;
    write(out_is_trusted, public_key == trusted_public_key)
}

fn write<T>(ptr: *mut T, value: T) -> Result<(), AvbIOError> {
    let ptr = to_nonnull(ptr)?;
    // SAFETY: It is safe as the raw pointer `ptr` is a nonnull pointer.
    unsafe {
        *ptr.as_ptr() = value;
    }
    Ok(())
}

fn as_ref<'a, T>(ptr: *mut T) -> Result<&'a T, AvbIOError> {
    let ptr = to_nonnull(ptr)?;
    // SAFETY: It is safe as the raw pointer `ptr` is a nonnull pointer.
    unsafe { Ok(ptr.as_ref()) }
}

fn to_nonnull<T>(ptr: *mut T) -> Result<NonNull<T>, AvbIOError> {
    NonNull::new(ptr).ok_or(AvbIOError::NoSuchValue)
}

fn is_not_null<T>(ptr: *const T) -> Result<(), AvbIOError> {
    if ptr.is_null() {
        Err(AvbIOError::NoSuchValue)
    } else {
        Ok(())
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
enum PartitionName {
    Kernel,
    InitrdNormal,
    InitrdDebug,
}

impl PartitionName {
    const KERNEL_PARTITION_NAME: &[u8] = b"boot\0";
    const INITRD_NORMAL_PARTITION_NAME: &[u8] = b"initrd_normal\0";
    const INITRD_DEBUG_PARTITION_NAME: &[u8] = b"initrd_debug\0";

    fn as_cstr(&self) -> &CStr {
        let partition_name = match self {
            Self::Kernel => Self::KERNEL_PARTITION_NAME,
            Self::InitrdNormal => Self::INITRD_NORMAL_PARTITION_NAME,
            Self::InitrdDebug => Self::INITRD_DEBUG_PARTITION_NAME,
        };
        CStr::from_bytes_with_nul(partition_name).unwrap()
    }
}

impl TryFrom<&CStr> for PartitionName {
    type Error = AvbIOError;

    fn try_from(partition_name: &CStr) -> Result<Self, Self::Error> {
        match partition_name.to_bytes_with_nul() {
            Self::KERNEL_PARTITION_NAME => Ok(Self::Kernel),
            Self::INITRD_NORMAL_PARTITION_NAME => Ok(Self::InitrdNormal),
            Self::INITRD_DEBUG_PARTITION_NAME => Ok(Self::InitrdDebug),
            _ => Err(AvbIOError::NoSuchPartition),
        }
    }
}

struct Payload<'a> {
    kernel: &'a [u8],
    initrd: Option<&'a [u8]>,
    trusted_public_key: &'a [u8],
}

impl<'a> AsRef<Payload<'a>> for AvbOps {
    fn as_ref(&self) -> &Payload<'a> {
        let payload = self.user_data as *const Payload;
        // SAFETY: It is safe to cast the `AvbOps.user_data` to Payload as we have saved a
        // pointer to a valid value of Payload in user_data when creating AvbOps, and
        // assume that the Payload isn't used beyond the lifetime of the AvbOps that it
        // belongs to.
        unsafe { &*payload }
    }
}

impl<'a> Payload<'a> {
    const MAX_NUM_OF_HASH_DESCRIPTORS: usize = 3;

    fn get_partition(&self, partition_name: *const c_char) -> Result<&[u8], AvbIOError> {
        is_not_null(partition_name)?;
        // SAFETY: It is safe as the raw pointer `partition_name` is a nonnull pointer.
        let partition_name = unsafe { CStr::from_ptr(partition_name) };
        match partition_name.try_into()? {
            PartitionName::Kernel => Ok(self.kernel),
            PartitionName::InitrdNormal | PartitionName::InitrdDebug => {
                self.initrd.ok_or(AvbIOError::NoSuchPartition)
            }
        }
    }

    fn verify_partitions(&mut self, partition_names: &[&CStr]) -> Result<(), AvbSlotVerifyError> {
        if partition_names.len() > Self::MAX_NUM_OF_HASH_DESCRIPTORS {
            return Err(AvbSlotVerifyError::InvalidArgument);
        }
        let mut requested_partitions = [ptr::null(); Self::MAX_NUM_OF_HASH_DESCRIPTORS + 1];
        partition_names
            .iter()
            .enumerate()
            .for_each(|(i, name)| requested_partitions[i] = name.as_ptr());

        let mut avb_ops = AvbOps {
            user_data: self as *mut _ as *mut c_void,
            ab_ops: ptr::null_mut(),
            atx_ops: ptr::null_mut(),
            read_from_partition: Some(read_from_partition),
            get_preloaded_partition: Some(get_preloaded_partition),
            write_to_partition: None,
            validate_vbmeta_public_key: None,
            read_rollback_index: Some(read_rollback_index),
            write_rollback_index: None,
            read_is_device_unlocked: Some(read_is_device_unlocked),
            get_unique_guid_for_partition: Some(get_unique_guid_for_partition),
            get_size_of_partition: Some(get_size_of_partition),
            read_persistent_value: None,
            write_persistent_value: None,
            validate_public_key_for_partition: Some(validate_public_key_for_partition),
        };
        let ab_suffix = CStr::from_bytes_with_nul(NULL_BYTE).unwrap();
        let out_data = ptr::null_mut();
        // SAFETY: It is safe to call `avb_slot_verify()` as the pointer arguments (`ops`,
        // `requested_partitions` and `ab_suffix`) passed to the method are all valid and
        // initialized. The last argument `out_data` is allowed to be null so that nothing
        // will be written to it.
        let result = unsafe {
            avb_slot_verify(
                &mut avb_ops,
                requested_partitions.as_ptr(),
                ab_suffix.as_ptr(),
                AvbSlotVerifyFlags::AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION,
                AvbHashtreeErrorMode::AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE,
                out_data,
            )
        };
        slot_verify_result_to_verify_payload_result(result)
    }
}

/// Verifies the payload (signed kernel + initrd) against the trusted public key.
pub fn verify_payload(
    kernel: &[u8],
    initrd: Option<&[u8]>,
    trusted_public_key: &[u8],
) -> Result<(), AvbSlotVerifyError> {
    let mut payload = Payload { kernel, initrd, trusted_public_key };
    let requested_partitions = [PartitionName::Kernel.as_cstr()];
    payload.verify_partitions(&requested_partitions)
}

#[cfg(test)]
mod tests {
    use super::*;
    use anyhow::Result;
    use avb_bindgen::AvbFooter;
    use std::{fs, mem::size_of};

    const MICRODROID_KERNEL_IMG_PATH: &str = "microdroid_kernel";
    const INITRD_NORMAL_IMG_PATH: &str = "microdroid_initrd_normal.img";
    const TEST_IMG_WITH_ONE_HASHDESC_PATH: &str = "test_image_with_one_hashdesc.img";
    const UNSIGNED_TEST_IMG_PATH: &str = "unsigned_test.img";

    const PUBLIC_KEY_RSA2048_PATH: &str = "data/testkey_rsa2048_pub.bin";
    const PUBLIC_KEY_RSA4096_PATH: &str = "data/testkey_rsa4096_pub.bin";
    const RANDOM_FOOTER_POS: usize = 30;

    /// This test uses the Microdroid payload compiled on the fly to check that
    /// the latest payload can be verified successfully.
    #[test]
    fn latest_valid_payload_passes_verification() -> Result<()> {
        let kernel = load_latest_signed_kernel()?;
        let initrd_normal = fs::read(INITRD_NORMAL_IMG_PATH)?;
        let public_key = fs::read(PUBLIC_KEY_RSA4096_PATH)?;

        assert_eq!(Ok(()), verify_payload(&kernel, Some(&initrd_normal[..]), &public_key));
        Ok(())
    }

    #[test]
    fn payload_expecting_no_initrd_passes_verification_with_no_initrd() -> Result<()> {
        let kernel = fs::read(TEST_IMG_WITH_ONE_HASHDESC_PATH)?;
        let public_key = fs::read(PUBLIC_KEY_RSA4096_PATH)?;

        assert_eq!(Ok(()), verify_payload(&kernel, None, &public_key));
        Ok(())
    }

    // TODO(b/256148034): Test that kernel with two hashdesc and no initrd fails verification.
    // e.g. payload_expecting_initrd_fails_verification_with_no_initrd

    #[test]
    fn payload_with_empty_public_key_fails_verification() -> Result<()> {
        assert_payload_verification_fails(
            &load_latest_signed_kernel()?,
            &load_latest_initrd_normal()?,
            /*trusted_public_key=*/ &[0u8; 0],
            AvbSlotVerifyError::PublicKeyRejected,
        )
    }

    #[test]
    fn payload_with_an_invalid_public_key_fails_verification() -> Result<()> {
        assert_payload_verification_fails(
            &load_latest_signed_kernel()?,
            &load_latest_initrd_normal()?,
            /*trusted_public_key=*/ &[0u8; 512],
            AvbSlotVerifyError::PublicKeyRejected,
        )
    }

    #[test]
    fn payload_with_a_different_valid_public_key_fails_verification() -> Result<()> {
        assert_payload_verification_fails(
            &load_latest_signed_kernel()?,
            &load_latest_initrd_normal()?,
            &fs::read(PUBLIC_KEY_RSA2048_PATH)?,
            AvbSlotVerifyError::PublicKeyRejected,
        )
    }

    #[test]
    fn unsigned_kernel_fails_verification() -> Result<()> {
        assert_payload_verification_fails(
            &fs::read(UNSIGNED_TEST_IMG_PATH)?,
            &load_latest_initrd_normal()?,
            &fs::read(PUBLIC_KEY_RSA4096_PATH)?,
            AvbSlotVerifyError::Io,
        )
    }

    #[test]
    fn tampered_kernel_fails_verification() -> Result<()> {
        let mut kernel = load_latest_signed_kernel()?;
        kernel[1] = !kernel[1]; // Flip the bits

        assert_payload_verification_fails(
            &kernel,
            &load_latest_initrd_normal()?,
            &fs::read(PUBLIC_KEY_RSA4096_PATH)?,
            AvbSlotVerifyError::Verification,
        )
    }

    #[test]
    fn tampered_kernel_footer_fails_verification() -> Result<()> {
        let mut kernel = load_latest_signed_kernel()?;
        let avb_footer_index = kernel.len() - size_of::<AvbFooter>() + RANDOM_FOOTER_POS;
        kernel[avb_footer_index] = !kernel[avb_footer_index];

        assert_payload_verification_fails(
            &kernel,
            &load_latest_initrd_normal()?,
            &fs::read(PUBLIC_KEY_RSA4096_PATH)?,
            AvbSlotVerifyError::InvalidMetadata,
        )
    }

    fn assert_payload_verification_fails(
        kernel: &[u8],
        initrd: &[u8],
        trusted_public_key: &[u8],
        expected_error: AvbSlotVerifyError,
    ) -> Result<()> {
        assert_eq!(Err(expected_error), verify_payload(kernel, Some(initrd), trusted_public_key));
        Ok(())
    }

    fn load_latest_signed_kernel() -> Result<Vec<u8>> {
        Ok(fs::read(MICRODROID_KERNEL_IMG_PATH)?)
    }

    fn load_latest_initrd_normal() -> Result<Vec<u8>> {
        Ok(fs::read(INITRD_NORMAL_IMG_PATH)?)
    }
}