guest/pvmfw/src/dice.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.

//! Support for DICE derivation and BCC generation.
extern crate alloc;

use alloc::format;
use alloc::vec::Vec;
use ciborium::cbor;
use ciborium::Value;
use core::mem::size_of;
use diced_open_dice::{
    bcc_handover_main_flow, hash, Config, DiceContext, DiceMode, Hash, InputValues, HIDDEN_SIZE,
};
use pvmfw_avb::{Capability, DebugLevel, Digest, VerifiedBootData};
use zerocopy::AsBytes;

// pVM firmware (like other VM components) is expected to populate some fields in DICE
// Configuration Descriptor. See dice_for_avf_guest.cddl
const COMPONENT_NAME_KEY: i64 = -70002;
const SECURITY_VERSION_KEY: i64 = -70005;
const RKP_VM_MARKER_KEY: i64 = -70006;
const INSTANCE_HASH_KEY: i64 = -71003;

#[derive(Debug)]
pub enum Error {
    /// Error in CBOR operations
    #[allow(dead_code)]
    CborError(ciborium::value::Error),
    /// Error in DICE operations
    #[allow(dead_code)]
    DiceError(diced_open_dice::DiceError),
}

impl From<ciborium::value::Error> for Error {
    fn from(e: ciborium::value::Error) -> Self {
        Self::CborError(e)
    }
}

impl From<diced_open_dice::DiceError> for Error {
    fn from(e: diced_open_dice::DiceError) -> Self {
        Self::DiceError(e)
    }
}

// DICE in pvmfw result type.
type Result<T> = core::result::Result<T, Error>;

fn to_dice_mode(debug_level: DebugLevel) -> DiceMode {
    match debug_level {
        DebugLevel::None => DiceMode::kDiceModeNormal,
        DebugLevel::Full => DiceMode::kDiceModeDebug,
    }
}

fn to_dice_hash(verified_boot_data: &VerifiedBootData) -> Result<Hash> {
    let mut digests = [0u8; size_of::<Digest>() * 2];
    digests[..size_of::<Digest>()].copy_from_slice(&verified_boot_data.kernel_digest);
    if let Some(initrd_digest) = verified_boot_data.initrd_digest {
        digests[size_of::<Digest>()..].copy_from_slice(&initrd_digest);
    }
    Ok(hash(&digests)?)
}

#[derive(Clone)]
pub struct PartialInputs {
    pub code_hash: Hash,
    pub auth_hash: Hash,
    pub mode: DiceMode,
    pub security_version: u64,
    pub rkp_vm_marker: bool,
}

impl PartialInputs {
    pub fn new(data: &VerifiedBootData) -> Result<Self> {
        let code_hash = to_dice_hash(data)?;
        let auth_hash = hash(data.public_key)?;
        let mode = to_dice_mode(data.debug_level);
        // We use rollback_index from vbmeta as the security_version field in dice certificate.
        let security_version = data.rollback_index;
        let rkp_vm_marker = data.has_capability(Capability::RemoteAttest);

        Ok(Self { code_hash, auth_hash, mode, security_version, rkp_vm_marker })
    }

    pub fn write_next_bcc(
        self,
        current_bcc_handover: &[u8],
        salt: &[u8; HIDDEN_SIZE],
        instance_hash: Option<Hash>,
        deferred_rollback_protection: bool,
        next_bcc: &mut [u8],
        context: DiceContext,
    ) -> Result<()> {
        let config = self
            .generate_config_descriptor(instance_hash)
            .map_err(|_| diced_open_dice::DiceError::InvalidInput)?;

        let dice_inputs = InputValues::new(
            self.code_hash,
            Config::Descriptor(&config),
            self.auth_hash,
            self.mode,
            self.make_hidden(salt, deferred_rollback_protection)?,
        );
        let _ = bcc_handover_main_flow(current_bcc_handover, &dice_inputs, next_bcc, context)?;
        Ok(())
    }

    fn make_hidden(
        &self,
        salt: &[u8; HIDDEN_SIZE],
        deferred_rollback_protection: bool,
    ) -> diced_open_dice::Result<[u8; HIDDEN_SIZE]> {
        // We want to make sure we get a different sealing CDI for:
        // - VMs with different salt values
        // - An RKP VM and any other VM (regardless of salt)
        // - depending on whether rollback protection has been deferred to payload. This ensures the
        //   adversary cannot leak the secrets by using old images & setting
        //   `deferred_rollback_protection` to true.
        // The hidden input for DICE affects the sealing CDI (but the values in the config
        // descriptor do not).
        // Since the hidden input has to be a fixed size, create it as a hash of the values we
        // want included.
        #[derive(AsBytes)]
        #[repr(C, packed)]
        struct HiddenInput {
            rkp_vm_marker: bool,
            salt: [u8; HIDDEN_SIZE],
            deferred_rollback_protection: bool,
        }
        hash(
            HiddenInput {
                rkp_vm_marker: self.rkp_vm_marker,
                salt: *salt,
                deferred_rollback_protection,
            }
            .as_bytes(),
        )
    }

    fn generate_config_descriptor(&self, instance_hash: Option<Hash>) -> Result<Vec<u8>> {
        let mut config = Vec::with_capacity(4);
        config.push((cbor!(COMPONENT_NAME_KEY)?, cbor!("vm_entry")?));
        if cfg!(dice_changes) {
            config.push((cbor!(SECURITY_VERSION_KEY)?, cbor!(self.security_version)?));
        }
        if self.rkp_vm_marker {
            config.push((cbor!(RKP_VM_MARKER_KEY)?, Value::Null))
        }
        if let Some(instance_hash) = instance_hash {
            config.push((cbor!(INSTANCE_HASH_KEY)?, Value::from(instance_hash.as_slice())));
        }
        let config = Value::Map(config);
        Ok(cbor_util::serialize(&config).map_err(|e| {
            ciborium::value::Error::Custom(format!("Error in serialization: {e:?}"))
        })?)
    }
}

#[cfg(test)]
mod tests {
    use crate::{
        Hash, PartialInputs, COMPONENT_NAME_KEY, INSTANCE_HASH_KEY, RKP_VM_MARKER_KEY,
        SECURITY_VERSION_KEY,
    };
    use ciborium::Value;
    use diced_open_dice::DiceArtifacts;
    use diced_open_dice::DiceContext;
    use diced_open_dice::DiceMode;
    use diced_open_dice::KeyAlgorithm;
    use diced_open_dice::HIDDEN_SIZE;
    use diced_sample_inputs::make_sample_bcc_and_cdis;
    use pvmfw_avb::Capability;
    use pvmfw_avb::DebugLevel;
    use pvmfw_avb::Digest;
    use pvmfw_avb::VerifiedBootData;
    use std::collections::HashMap;
    use std::mem::size_of;
    use std::vec;

    const COMPONENT_VERSION_KEY: i64 = -70003;
    const RESETTABLE_KEY: i64 = -70004;
    const BASE_VB_DATA: VerifiedBootData = VerifiedBootData {
        debug_level: DebugLevel::None,
        kernel_digest: [1u8; size_of::<Digest>()],
        initrd_digest: Some([2u8; size_of::<Digest>()]),
        public_key: b"public key",
        capabilities: vec![],
        rollback_index: 42,
    };
    const HASH: Hash = *b"sixtyfourbyteslongsentencearerarebutletsgiveitatrycantbethathard";

    #[test]
    fn base_data_conversion() {
        let vb_data = BASE_VB_DATA;
        let inputs = PartialInputs::new(&vb_data).unwrap();

        assert_eq!(inputs.mode, DiceMode::kDiceModeNormal);
        assert_eq!(inputs.security_version, 42);
        assert!(!inputs.rkp_vm_marker);

        // TODO(b/313608219): Consider checks for code_hash and possibly auth_hash.
    }

    #[test]
    fn debuggable_conversion() {
        let vb_data = VerifiedBootData { debug_level: DebugLevel::Full, ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();

        assert_eq!(inputs.mode, DiceMode::kDiceModeDebug);
    }

    #[test]
    fn rkp_vm_conversion() {
        let vb_data =
            VerifiedBootData { capabilities: vec![Capability::RemoteAttest], ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();

        assert!(inputs.rkp_vm_marker);
    }

    #[test]
    fn base_config_descriptor() {
        let vb_data = BASE_VB_DATA;
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let config_map = decode_config_descriptor(&inputs, None);

        assert_eq!(config_map.get(&COMPONENT_NAME_KEY).unwrap().as_text().unwrap(), "vm_entry");
        assert_eq!(config_map.get(&COMPONENT_VERSION_KEY), None);
        assert_eq!(config_map.get(&RESETTABLE_KEY), None);
        if cfg!(dice_changes) {
            assert_eq!(
                config_map.get(&SECURITY_VERSION_KEY).unwrap().as_integer().unwrap(),
                42.into()
            );
        } else {
            assert_eq!(config_map.get(&SECURITY_VERSION_KEY), None);
        }
        assert_eq!(config_map.get(&RKP_VM_MARKER_KEY), None);
    }

    #[test]
    fn config_descriptor_with_rkp_vm() {
        let vb_data =
            VerifiedBootData { capabilities: vec![Capability::RemoteAttest], ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let config_map = decode_config_descriptor(&inputs, Some(HASH));

        assert!(config_map.get(&RKP_VM_MARKER_KEY).unwrap().is_null());
    }

    #[test]
    fn config_descriptor_with_instance_hash() {
        let vb_data =
            VerifiedBootData { capabilities: vec![Capability::RemoteAttest], ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let config_map = decode_config_descriptor(&inputs, Some(HASH));
        assert_eq!(*config_map.get(&INSTANCE_HASH_KEY).unwrap(), Value::from(HASH.as_slice()));
    }

    #[test]
    fn config_descriptor_without_instance_hash() {
        let vb_data =
            VerifiedBootData { capabilities: vec![Capability::RemoteAttest], ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let config_map = decode_config_descriptor(&inputs, None);
        assert!(!config_map.contains_key(&INSTANCE_HASH_KEY));
    }

    fn decode_config_descriptor(
        inputs: &PartialInputs,
        instance_hash: Option<Hash>,
    ) -> HashMap<i64, Value> {
        let config_descriptor = inputs.generate_config_descriptor(instance_hash).unwrap();

        let cbor_map =
            cbor_util::deserialize::<Value>(&config_descriptor).unwrap().into_map().unwrap();

        cbor_map
            .into_iter()
            .map(|(k, v)| ((k.into_integer().unwrap().try_into().unwrap()), v))
            .collect()
    }

    #[test]
    fn changing_deferred_rpb_changes_secrets() {
        let vb_data = VerifiedBootData { debug_level: DebugLevel::Full, ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let mut buffer_without_defer = [0; 4096];
        let mut buffer_with_defer = [0; 4096];
        let mut buffer_without_defer_retry = [0; 4096];
        let context = DiceContext {
            authority_algorithm: KeyAlgorithm::Ed25519,
            subject_algorithm: KeyAlgorithm::Ed25519,
        };

        let sample_dice_input: &[u8] = &[
            0xa3, // CDI attest
            0x01, 0x58, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // CDI seal
            0x02, 0x58, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // DICE chain
            0x03, 0x82, 0xa6, 0x01, 0x02, 0x03, 0x27, 0x04, 0x02, 0x20, 0x01, 0x21, 0x40, 0x22,
            0x40, 0x84, 0x40, 0xa0, 0x40, 0x40,
            // 8-bytes of trailing data that aren't part of the DICE chain.
            0x84, 0x41, 0x55, 0xa0, 0x42, 0x11, 0x22, 0x40,
        ];

        inputs
            .clone()
            .write_next_bcc(
                sample_dice_input,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                false,
                &mut buffer_without_defer,
                context.clone(),
            )
            .unwrap();
        let bcc_handover1 = diced_open_dice::bcc_handover_parse(&buffer_without_defer).unwrap();

        inputs
            .clone()
            .write_next_bcc(
                sample_dice_input,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                true,
                &mut buffer_with_defer,
                context.clone(),
            )
            .unwrap();
        let bcc_handover2 = diced_open_dice::bcc_handover_parse(&buffer_with_defer).unwrap();

        inputs
            .clone()
            .write_next_bcc(
                sample_dice_input,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                false,
                &mut buffer_without_defer_retry,
                context.clone(),
            )
            .unwrap();
        let bcc_handover3 =
            diced_open_dice::bcc_handover_parse(&buffer_without_defer_retry).unwrap();

        assert_ne!(bcc_handover1.cdi_seal(), bcc_handover2.cdi_seal());
        assert_eq!(bcc_handover1.cdi_seal(), bcc_handover3.cdi_seal());
    }

    #[test]
    fn dice_derivation_with_different_algorithms_is_valid() {
        let dice_artifacts = make_sample_bcc_and_cdis().unwrap();
        let bcc_handover0_bytes = to_bcc_handover(&dice_artifacts);
        let vb_data = VerifiedBootData { debug_level: DebugLevel::Full, ..BASE_VB_DATA };
        let inputs = PartialInputs::new(&vb_data).unwrap();
        let mut buffer = [0; 4096];

        inputs
            .clone()
            .write_next_bcc(
                &bcc_handover0_bytes,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                true,
                &mut buffer,
                DiceContext {
                    authority_algorithm: KeyAlgorithm::Ed25519,
                    subject_algorithm: KeyAlgorithm::EcdsaP256,
                },
            )
            .expect("Failed to derive Ed25519 -> EcdsaP256 BCC");
        let bcc_handover1 = diced_open_dice::bcc_handover_parse(&buffer).unwrap();
        let bcc_handover1_bytes = to_bcc_handover(&bcc_handover1);
        buffer.fill(0);

        inputs
            .clone()
            .write_next_bcc(
                &bcc_handover1_bytes,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                true,
                &mut buffer,
                DiceContext {
                    authority_algorithm: KeyAlgorithm::EcdsaP256,
                    subject_algorithm: KeyAlgorithm::EcdsaP384,
                },
            )
            .expect("Failed to derive EcdsaP256 -> EcdsaP384 BCC");
        let bcc_handover2 = diced_open_dice::bcc_handover_parse(&buffer).unwrap();
        let bcc_handover2_bytes = to_bcc_handover(&bcc_handover2);
        buffer.fill(0);

        inputs
            .clone()
            .write_next_bcc(
                &bcc_handover2_bytes,
                &[0u8; HIDDEN_SIZE],
                Some([0u8; 64]),
                true,
                &mut buffer,
                DiceContext {
                    authority_algorithm: KeyAlgorithm::EcdsaP384,
                    subject_algorithm: KeyAlgorithm::Ed25519,
                },
            )
            .expect("Failed to derive EcdsaP384 -> Ed25519 BCC");
        let _bcc_handover3 = diced_open_dice::bcc_handover_parse(&buffer).unwrap();

        // TODO(b/378813154): Check the DICE chain with `hwtrust` once the profile version
        // is updated.
        // The check cannot be done now because parsing the chain causes the following error:
        // Invalid payload at index 3. Caused by:
        // 0: opendice.example.p256
        // 1: unknown profile version
    }

    fn to_bcc_handover(dice_artifacts: &dyn DiceArtifacts) -> Vec<u8> {
        let dice_chain = cbor_util::deserialize::<Value>(dice_artifacts.bcc().unwrap()).unwrap();
        let bcc_handover = Value::Map(vec![
            (Value::Integer(1.into()), Value::Bytes(dice_artifacts.cdi_attest().to_vec())),
            (Value::Integer(2.into()), Value::Bytes(dice_artifacts.cdi_seal().to_vec())),
            (Value::Integer(3.into()), dice_chain),
        ]);
        cbor_util::serialize(&bcc_handover).unwrap()
    }
}