guest/pvmfw/src/rollback.rs

// Copyright 2024, 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 guest-specific rollback protection (RBP).

use crate::dice::PartialInputs;
use crate::entry::RebootReason;
use crate::fdt::read_defer_rollback_protection;
use crate::instance::EntryBody;
use crate::instance::Error as InstanceError;
use crate::instance::{get_recorded_entry, record_instance_entry};
use diced_open_dice::Hidden;
use libfdt::Fdt;
use log::{error, info};
use pvmfw_avb::Capability;
use pvmfw_avb::VerifiedBootData;
use virtio_drivers::transport::pci::bus::PciRoot;
use vmbase::rand;

/// Performs RBP based on the input payload, current DICE chain, and host-controlled platform.
///
/// On success, returns a tuple containing:
/// - `new_instance`: true if the legacy instance.img solution was used and a new entry created;
/// - `salt`: the salt representing the instance, to be used during DICE derivation;
/// - `defer_rollback_protection`: if RBP is being deferred.
pub fn perform_rollback_protection(
    fdt: &Fdt,
    verified_boot_data: &VerifiedBootData,
    dice_inputs: &PartialInputs,
    pci_root: &mut PciRoot,
    cdi_seal: &[u8],
    instance_hash: Option<Hidden>,
) -> Result<(bool, Hidden, bool), RebootReason> {
    if should_defer_rollback_protection(fdt)?
        && verified_boot_data.has_capability(Capability::SecretkeeperProtection)
    {
        perform_deferred_rollback_protection(verified_boot_data)?;
        Ok((false, instance_hash.unwrap(), true))
    } else if verified_boot_data.has_capability(Capability::RemoteAttest) {
        perform_fixed_index_rollback_protection(verified_boot_data)?;
        Ok((false, instance_hash.unwrap(), false))
    } else if verified_boot_data.has_capability(Capability::TrustySecurityVm) {
        skip_rollback_protection()?;
        Ok((false, instance_hash.unwrap(), false))
    } else {
        perform_legacy_rollback_protection(dice_inputs, pci_root, cdi_seal, instance_hash)
    }
}

fn perform_deferred_rollback_protection(
    verified_boot_data: &VerifiedBootData,
) -> Result<(), RebootReason> {
    info!("Deferring rollback protection");
    // rollback_index of the image is used as security_version and is expected to be > 0 to
    // discourage implicit allocation.
    if verified_boot_data.rollback_index == 0 {
        error!("Expected positive rollback_index, found 0");
        Err(RebootReason::InvalidPayload)
    } else {
        Ok(())
    }
}

fn perform_fixed_index_rollback_protection(
    verified_boot_data: &VerifiedBootData,
) -> Result<(), RebootReason> {
    info!("Performing fixed-index rollback protection");
    let fixed_index = service_vm_version::VERSION;
    let index = verified_boot_data.rollback_index;
    if index != fixed_index {
        error!("Rollback index mismatch: expected {fixed_index}, found {index}");
        Err(RebootReason::InvalidPayload)
    } else {
        Ok(())
    }
}

fn skip_rollback_protection() -> Result<(), RebootReason> {
    info!("Skipping rollback protection");
    Ok(())
}

/// Performs RBP using instance.img where updates require clearing old entries, causing new CDIs.
fn perform_legacy_rollback_protection(
    dice_inputs: &PartialInputs,
    pci_root: &mut PciRoot,
    cdi_seal: &[u8],
    instance_hash: Option<Hidden>,
) -> Result<(bool, Hidden, bool), RebootReason> {
    info!("Fallback to instance.img based rollback checks");
    let (recorded_entry, mut instance_img, header_index) = get_recorded_entry(pci_root, cdi_seal)
        .map_err(|e| {
        error!("Failed to get entry from instance.img: {e}");
        RebootReason::InternalError
    })?;
    let (new_instance, salt) = if let Some(entry) = recorded_entry {
        check_dice_measurements_match_entry(dice_inputs, &entry)?;
        let salt = instance_hash.unwrap_or(entry.salt);
        (false, salt)
    } else {
        // New instance!
        let salt = instance_hash.map_or_else(rand::random_array, Ok).map_err(|e| {
            error!("Failed to generated instance.img salt: {e}");
            RebootReason::InternalError
        })?;

        let entry = EntryBody::new(dice_inputs, &salt);
        record_instance_entry(&entry, cdi_seal, &mut instance_img, header_index).map_err(|e| {
            error!("Failed to get recorded entry in instance.img: {e}");
            RebootReason::InternalError
        })?;
        (true, salt)
    };
    Ok((new_instance, salt, false))
}

fn check_dice_measurements_match_entry(
    dice_inputs: &PartialInputs,
    entry: &EntryBody,
) -> Result<(), RebootReason> {
    ensure_dice_measurements_match_entry(dice_inputs, entry).map_err(|e| {
        error!(
            "Dice measurements do not match recorded entry. \
        This may be because of update: {e}"
        );
        RebootReason::InternalError
    })?;

    Ok(())
}

fn ensure_dice_measurements_match_entry(
    dice_inputs: &PartialInputs,
    entry: &EntryBody,
) -> Result<(), InstanceError> {
    if entry.code_hash != dice_inputs.code_hash {
        Err(InstanceError::RecordedCodeHashMismatch)
    } else if entry.auth_hash != dice_inputs.auth_hash {
        Err(InstanceError::RecordedAuthHashMismatch)
    } else if entry.mode() != dice_inputs.mode {
        Err(InstanceError::RecordedDiceModeMismatch)
    } else {
        Ok(())
    }
}

fn should_defer_rollback_protection(fdt: &Fdt) -> Result<bool, RebootReason> {
    let defer_rbp = read_defer_rollback_protection(fdt).map_err(|e| {
        error!("Failed to get defer-rollback-protection property in DT: {e}");
        RebootReason::InvalidFdt
    })?;
    Ok(defer_rbp.is_some())
}