pw_software_update/update_bundle_accessor.cc

// Copyright 2021 The Pigweed Authors
//
// 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
//
//     https://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.

#include "pw_software_update/update_bundle_accessor.h"

#include <cstddef>
#include <cstring>
#include <string_view>

#include "pw_crypto/ecdsa.h"
#include "pw_crypto/sha256.h"
#include "pw_log/log.h"
#include "pw_protobuf/message.h"
#include "pw_result/result.h"
#include "pw_software_update/config.h"
#include "pw_software_update/update_bundle.pwpb.h"
#include "pw_stream/interval_reader.h"
#include "pw_stream/memory_stream.h"

#define PW_LOG_LEVEL PW_SOFTWARE_UPDATE_CONFIG_LOG_LEVEL

namespace pw::software_update {
namespace {

constexpr std::string_view kTopLevelTargetsName = "targets";

Result<bool> VerifyEcdsaSignature(protobuf::Bytes public_key,
                                  ConstByteSpan digest,
                                  protobuf::Bytes signature) {
  // TODO(pwbug/456): Move this logic into an variant of the API in
  // pw_crypto:ecdsa that takes readers as inputs.
  std::byte public_key_bytes[65];
  std::byte signature_bytes[64];
  stream::IntervalReader key_reader = public_key.GetBytesReader();
  stream::IntervalReader sig_reader = signature.GetBytesReader();
  PW_TRY(key_reader.Read(public_key_bytes));
  PW_TRY(sig_reader.Read(signature_bytes));
  Status status = crypto::ecdsa::VerifyP256Signature(
      public_key_bytes, digest, signature_bytes);
  if (!status.ok()) {
    return false;
  }

  return true;
}

// Convert an integer from [0, 16) to a hex char
char IntToHex(uint8_t val) {
  PW_ASSERT(val < 16);
  return val >= 10 ? (val - 10) + 'a' : val + '0';
}

void LogKeyId(ConstByteSpan key_id) {
  char key_id_str[pw::crypto::sha256::kDigestSizeBytes * 2 + 1] = {0};
  for (size_t i = 0; i < pw::crypto::sha256::kDigestSizeBytes; i++) {
    uint8_t value = std::to_integer<uint8_t>(key_id[i]);
    key_id_str[i * 2] = IntToHex((value >> 4) & 0xf);
    key_id_str[i * 2 + 1] = IntToHex(value & 0xf);
  }

  PW_LOG_DEBUG("key_id: %s", key_id_str);
}

// Verifies signatures of a TUF metadata.
Result<bool> VerifyTufMetadataSignatures(
    protobuf::Bytes message,
    protobuf::RepeatedMessages signatures,
    protobuf::Message signature_requirement,
    protobuf::StringToMessageMap key_mapping) {
  // Gets the threshold -- at least `threshold` number of signatures must
  // pass verification in order to trust this metadata.
  protobuf::Uint32 threshold = signature_requirement.AsUint32(
      static_cast<uint32_t>(SignatureRequirement::Fields::THRESHOLD));
  PW_TRY(threshold.status());

  // Gets the ids of keys that are allowed for verifying the signatures.
  protobuf::RepeatedBytes allowed_key_ids =
      signature_requirement.AsRepeatedBytes(
          static_cast<uint32_t>(SignatureRequirement::Fields::KEY_IDS));
  PW_TRY(allowed_key_ids.status());

  // Verifies the signatures. Check that at least `threshold` number of
  // signatures can be verified using the allowed keys.
  size_t verified_count = 0;
  for (protobuf::Message signature : signatures) {
    protobuf::Bytes key_id =
        signature.AsBytes(static_cast<uint32_t>(Signature::Fields::KEY_ID));
    PW_TRY(key_id.status());

    // Reads the key id into a buffer, so that we can check whether it is
    // listed as allowed and look up the key value later.
    std::byte key_id_buf[pw::crypto::sha256::kDigestSizeBytes];
    stream::IntervalReader key_id_reader = key_id.GetBytesReader();
    Result<ByteSpan> key_id_read_res = key_id_reader.Read(key_id_buf);
    PW_TRY(key_id_read_res.status());
    if (key_id_read_res.value().size() != sizeof(key_id_buf)) {
      return Status::Internal();
    }

    // Verify that the `key_id` is listed in `allowed_key_ids`.
    // Note that the function assumes that the key id is properly derived
    // from the key (via sha256).
    bool key_id_is_allowed = false;
    for (protobuf::Bytes trusted : allowed_key_ids) {
      Result<bool> key_id_equal = trusted.Equal(key_id_buf);
      PW_TRY(key_id_equal.status());
      if (key_id_equal.value()) {
        key_id_is_allowed = true;
        break;
      }
    }

    if (!key_id_is_allowed) {
      PW_LOG_DEBUG("Skipping a key id not listed in allowed key ids.");
      LogKeyId(key_id_buf);
      continue;
    }

    // Retrieves the signature bytes.
    protobuf::Bytes sig =
        signature.AsBytes(static_cast<uint32_t>(Signature::Fields::SIG));
    PW_TRY(sig.status());

    // Extracts the key type, scheme and value information.
    std::string_view key_id_str(reinterpret_cast<const char*>(key_id_buf),
                                sizeof(key_id_buf));
    protobuf::Message key_info = key_mapping[key_id_str];
    PW_TRY(key_info.status());

    protobuf::Bytes key_val =
        key_info.AsBytes(static_cast<uint32_t>(Key::Fields::KEYVAL));
    PW_TRY(key_val.status());

    // The function assume that all keys are ECDSA keys. This is guaranteed
    // by the fact that all trusted roots have undergone content check.

    // computes the sha256 hash
    std::byte sha256_digest[32];
    stream::IntervalReader bytes_reader = message.GetBytesReader();
    PW_TRY(crypto::sha256::Hash(bytes_reader, sha256_digest));
    Result<bool> res = VerifyEcdsaSignature(key_val, sha256_digest, sig);
    PW_TRY(res.status());
    if (res.value()) {
      verified_count++;
      if (verified_count == threshold.value()) {
        return true;
      }
    }
  }

  PW_LOG_DEBUG(
      "Not enough number of signatures verified. Requires at least %u, "
      "verified %u",
      threshold.value(),
      verified_count);
  return false;
}

// Verifies the signatures of a signed new root metadata against a given
// trusted root. The helper function extracts the corresponding key maping
// signature requirement, signatures from the trusted root and passes them
// to VerifyTufMetadataSignatures().
//
// Precondition: The trusted root metadata has undergone content validity check.
Result<bool> VerifyRootMetadataSignatures(protobuf::Message trusted_root,
                                          protobuf::Message new_root) {
  // Retrieves the trusted root metadata content message.
  protobuf::Message trusted = trusted_root.AsMessage(static_cast<uint32_t>(
      SignedRootMetadata::Fields::SERIALIZED_ROOT_METADATA));
  PW_TRY(trusted.status());

  // Retrieves the serialized new root metadata bytes.
  protobuf::Bytes serialized = new_root.AsBytes(static_cast<uint32_t>(
      SignedRootMetadata::Fields::SERIALIZED_ROOT_METADATA));
  PW_TRY(serialized.status());

  // Gets the key mapping from the trusted root metadata.
  protobuf::StringToMessageMap key_mapping = trusted.AsStringToMessageMap(
      static_cast<uint32_t>(RootMetadata::Fields::KEYS));
  PW_TRY(key_mapping.status());

  // Gets the signatures of the new root.
  protobuf::RepeatedMessages signatures = new_root.AsRepeatedMessages(
      static_cast<uint32_t>(SignedRootMetadata::Fields::SIGNATURES));
  PW_TRY(signatures.status());

  // Gets the signature requirement from the trusted root metadata.
  protobuf::Message signature_requirement = trusted.AsMessage(
      static_cast<uint32_t>(RootMetadata::Fields::ROOT_SIGNATURE_REQUIREMENT));
  PW_TRY(signature_requirement.status());

  // Verifies the signatures.
  return VerifyTufMetadataSignatures(
      serialized, signatures, signature_requirement, key_mapping);
}

}  // namespace

Status UpdateBundleAccessor::OpenAndVerify(const ManifestAccessor&) {
  PW_TRY(DoOpen());
#if !defined(PW_SOFTWARE_UPDATE_LANDING_BUNDLE_VERIFICATION)
  PW_TRY(DoVerify());
#endif
  return OkStatus();
}

// Get the target element corresponding to `target_file`
stream::IntervalReader UpdateBundleAccessor::GetTargetPayload(
    std::string_view target_file_name) {
  protobuf::StringToBytesMap target_payloads =
      decoder_.AsStringToBytesMap(static_cast<uint32_t>(
          pw::software_update::UpdateBundle::Fields::TARGET_PAYLOADS));
  PW_TRY(target_payloads.status());
  protobuf::Bytes payload = target_payloads[target_file_name];
  PW_TRY(payload.status());
  return payload.GetBytesReader();
}

Result<bool> UpdateBundleAccessor::IsTargetPayloadIncluded(
    std::string_view target_file_name) {
  // TODO(pwbug/456): Perform personalization check first. If the target
  // is personalized out. Don't need to proceed.

  protobuf::StringToMessageMap signed_targets_metadata_map =
      decoder_.AsStringToMessageMap(static_cast<uint32_t>(
          pw::software_update::UpdateBundle::Fields::TARGETS_METADATA));
  PW_TRY(signed_targets_metadata_map.status());

  // There should only be one element in the map, which is the top-level
  // targets metadata.
  protobuf::Message signed_targets_metadata =
      signed_targets_metadata_map[kTopLevelTargetsName];
  PW_TRY(signed_targets_metadata.status());

  protobuf::Message metadata = signed_targets_metadata.AsMessage(
      static_cast<uint32_t>(pw::software_update::SignedTargetsMetadata::Fields::
                                SERIALIZED_TARGETS_METADATA));
  PW_TRY(metadata.status());

  protobuf::RepeatedMessages target_files =
      metadata.AsRepeatedMessages(static_cast<uint32_t>(
          pw::software_update::TargetsMetadata::Fields::TARGET_FILES));
  PW_TRY(target_files.status());

  for (protobuf::Message target_file : target_files) {
    protobuf::String name = target_file.AsString(static_cast<uint32_t>(
        pw::software_update::TargetFile::Fields::FILE_NAME));
    PW_TRY(name.status());
    Result<bool> file_name_matches = name.Equal(target_file_name);
    PW_TRY(file_name_matches.status());
    if (file_name_matches.value()) {
      return true;
    }
  }

  return false;
}

Status UpdateBundleAccessor::WriteManifest(
    [[maybe_unused]] stream::Writer& staged_manifest_writer) {
  return Status::Unimplemented();
}

Status UpdateBundleAccessor::Close() {
  // TODO(pwbug/456): To be implemented.
  return bundle_reader_.Close();
}

Status UpdateBundleAccessor::DoOpen() {
  PW_TRY(bundle_.Init());
  PW_TRY(bundle_reader_.Open());
  decoder_ =
      protobuf::Message(bundle_reader_, bundle_reader_.ConservativeReadLimit());
  (void)backend_;
  return OkStatus();
}

Status UpdateBundleAccessor::DoVerify() {
  if (disable_verification_) {
    PW_LOG_WARN("Update bundle verification is disabled.");
    return OkStatus();
  }

  // Verify and upgrade the on-device trust to the incoming root metadata if
  // one is included.
  PW_TRY(DoUpgradeRoot());

  // TODO(pwbug/456): Verify the targets metadata against the current trusted
  // root.

  // TODO(pwbug/456): Investigate whether targets payload verification should
  // be performed here or deferred until a specific target is requested.

  // TODO(pwbug/456): Invoke the backend to do downstream verification of the
  // bundle (e.g. compatibility and manifest completeness checks).

  return OkStatus();
}

Status UpdateBundleAccessor::DoUpgradeRoot() {
  protobuf::Message new_root = decoder_.AsMessage(
      static_cast<uint32_t>(UpdateBundle::Fields::ROOT_METADATA));
  if (new_root.status().IsNotFound()) {
    return OkStatus();
  }

  PW_TRY(new_root.status());

  // Get the trusted root and prepare for verification.
  Result<stream::SeekableReader*> trusted_root_reader =
      backend_.GetRootMetadataReader();
  PW_TRY(trusted_root_reader.status());
  PW_CHECK_NOTNULL(trusted_root_reader.value());
  protobuf::Message trusted_root(
      *trusted_root_reader.value(),
      trusted_root_reader.value()->ConservativeReadLimit());

  // TODO(pwbug/456): Check whether the bundle contains a root metadata that
  // is different from the on-device trusted root.

  // Verify the signatures against the trusted root metadata.
  Result<bool> verify_res =
      VerifyRootMetadataSignatures(trusted_root, new_root);
  PW_TRY(verify_res.status());
  if (!verify_res.value()) {
    PW_LOG_INFO("Fail to verify signatures against the current root");
    return Status::Unauthenticated();
  }

  // TODO(pwbug/456): Verifiy the content of the new root metadata, including:
  //    1) Check role magic field.
  //    2) Check signature requirement. Specifically, check that no key is
  //       reused across different roles and keys are unique in the same
  //       requirement.
  //    3) Check key mapping. Specifically, check that all keys are unique,
  //       ECDSA keys, and the key ids are exactly the SHA256 of `key type +
  //       key scheme + key value`.

  // Verify the signatures against the new root metadata.
  verify_res = VerifyRootMetadataSignatures(new_root, new_root);
  PW_TRY(verify_res.status());
  if (!verify_res.value()) {
    PW_LOG_INFO("Fail to verify signatures against the new root");
    return Status::Unauthenticated();
  }

  // TODO(pwbug/456): Check rollback.

  // Persist the new root.
  stream::IntervalReader new_root_reader = new_root.ToBytes().GetBytesReader();
  PW_TRY(backend_.SafelyPersistRootMetadata(new_root_reader));

  // TODO(pwbug/456): Implement key change detection to determine whether
  // rotation has occured or not. Delete the persisted targets metadata version
  // if any of the targets keys has been rotated.

  return OkStatus();
}

}  // namespace pw::software_update