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::instance::EntryBody;
use crate::instance::Error as InstanceError;
use crate::instance::{get_recorded_entry, record_instance_entry};
use cstr::cstr;
use diced_open_dice::Hidden;
use libfdt::{Fdt, FdtNode};
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 a new entry was created using the legacy instance.img solution;
/// - `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> {
    let defer_rollback_protection = should_defer_rollback_protection(fdt)?
        && verified_boot_data.has_capability(Capability::SecretkeeperProtection);
    let (new_instance, salt) = if defer_rollback_protection {
        info!("Guest OS is capable of Secretkeeper protection, 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");
            return Err(RebootReason::InvalidPayload);
        };
        (false, instance_hash.unwrap())
    } else if verified_boot_data.has_capability(Capability::RemoteAttest) {
        info!("Service VM capable of remote attestation detected, performing version checks");
        if service_vm_version::VERSION != verified_boot_data.rollback_index {
            // For RKP VM, we only boot if the version in the AVB footer of its kernel matches
            // the one embedded in pvmfw at build time.
            // This prevents the pvmfw from booting a roll backed RKP VM.
            error!(
                "Service VM version mismatch: expected {}, found {}",
                service_vm_version::VERSION,
                verified_boot_data.rollback_index
            );
            return Err(RebootReason::InvalidPayload);
        }
        (false, instance_hash.unwrap())
    } else if verified_boot_data.has_capability(Capability::TrustySecurityVm) {
        // The rollback protection of Trusty VMs are handled by AuthMgr, so we don't need to
        // handle it here.
        info!("Trusty Security VM detected");
        (false, instance_hash.unwrap())
    } else {
        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)
        };
        (new_instance, salt)
    };

    Ok((new_instance, salt, defer_rollback_protection))
}

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 node = avf_untrusted_node(fdt)?;
    let defer_rbp = node
        .getprop(cstr!("defer-rollback-protection"))
        .map_err(|e| {
            error!("Failed to get defer-rollback-protection property in DT: {e}");
            RebootReason::InvalidFdt
        })?
        .is_some();
    Ok(defer_rbp)
}

fn avf_untrusted_node(fdt: &Fdt) -> Result<FdtNode, RebootReason> {
    let node = fdt.node(cstr!("/avf/untrusted")).map_err(|e| {
        error!("Failed to get /avf/untrusted node: {e}");
        RebootReason::InvalidFdt
    })?;
    node.ok_or_else(|| {
        error!("/avf/untrusted node is missing in DT");
        RebootReason::InvalidFdt
    })
}