KeyUtil.cpp

/*
 * Copyright (C) 2016 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.
 */

#include "KeyUtil.h"

#include <iomanip>
#include <sstream>
#include <string>

#include <fcntl.h>
#include <linux/fscrypt.h>
#include <openssl/sha.h>
#include <sys/ioctl.h>

#include <android-base/file.h>
#include <android-base/logging.h>
#include <keyutils.h>

#include "KeyStorage.h"
#include "Utils.h"

namespace android {
namespace vold {

constexpr int FS_AES_256_XTS_KEY_SIZE = 64;

bool randomKey(KeyBuffer* key) {
    *key = KeyBuffer(FS_AES_256_XTS_KEY_SIZE);
    if (ReadRandomBytes(key->size(), key->data()) != 0) {
        // TODO status_t plays badly with PLOG, fix it.
        LOG(ERROR) << "Random read failed";
        return false;
    }
    return true;
}

// Return true if the kernel supports the ioctls to add/remove fscrypt keys
// directly to/from the filesystem.
bool isFsKeyringSupported(void) {
    static bool initialized = false;
    static bool supported;

    if (!initialized) {
        android::base::unique_fd fd(open("/data", O_RDONLY | O_DIRECTORY | O_CLOEXEC));

        // FS_IOC_ADD_ENCRYPTION_KEY with a NULL argument will fail with ENOTTY
        // if the ioctl isn't supported.  Otherwise it will fail with another
        // error code such as EFAULT.
        errno = 0;
        (void)ioctl(fd, FS_IOC_ADD_ENCRYPTION_KEY, NULL);
        if (errno == ENOTTY) {
            LOG(INFO) << "Kernel doesn't support FS_IOC_ADD_ENCRYPTION_KEY.  Falling back to "
                         "session keyring";
            supported = false;
        } else {
            if (errno != EFAULT) {
                PLOG(WARNING) << "Unexpected error from FS_IOC_ADD_ENCRYPTION_KEY";
            }
            LOG(DEBUG) << "Detected support for FS_IOC_ADD_ENCRYPTION_KEY";
            supported = true;
        }
        // There's no need to check for FS_IOC_REMOVE_ENCRYPTION_KEY, since it's
        // guaranteed to be available if FS_IOC_ADD_ENCRYPTION_KEY is.  There's
        // also no need to check for support on external volumes separately from
        // /data, since either the kernel supports the ioctls on all
        // fscrypt-capable filesystems or it doesn't.

        initialized = true;
    }
    return supported;
}

// Get raw keyref - used to make keyname and to pass to ioctl
static std::string generateKeyRef(const uint8_t* key, int length) {
    SHA512_CTX c;

    SHA512_Init(&c);
    SHA512_Update(&c, key, length);
    unsigned char key_ref1[SHA512_DIGEST_LENGTH];
    SHA512_Final(key_ref1, &c);

    SHA512_Init(&c);
    SHA512_Update(&c, key_ref1, SHA512_DIGEST_LENGTH);
    unsigned char key_ref2[SHA512_DIGEST_LENGTH];
    SHA512_Final(key_ref2, &c);

    static_assert(FS_KEY_DESCRIPTOR_SIZE <= SHA512_DIGEST_LENGTH, "Hash too short for descriptor");
    return std::string((char*)key_ref2, FS_KEY_DESCRIPTOR_SIZE);
}

static bool fillKey(const KeyBuffer& key, fscrypt_key* fs_key) {
    if (key.size() != FS_AES_256_XTS_KEY_SIZE) {
        LOG(ERROR) << "Wrong size key " << key.size();
        return false;
    }
    static_assert(FS_AES_256_XTS_KEY_SIZE <= sizeof(fs_key->raw), "Key too long!");
    fs_key->mode = FS_ENCRYPTION_MODE_AES_256_XTS;
    fs_key->size = key.size();
    memset(fs_key->raw, 0, sizeof(fs_key->raw));
    memcpy(fs_key->raw, key.data(), key.size());
    return true;
}

static char const* const NAME_PREFIXES[] = {"ext4", "f2fs", "fscrypt", nullptr};

static std::string keyrefstring(const std::string& raw_ref) {
    std::ostringstream o;
    for (unsigned char i : raw_ref) {
        o << std::hex << std::setw(2) << std::setfill('0') << (int)i;
    }
    return o.str();
}

static std::string buildLegacyKeyName(const std::string& prefix, const std::string& raw_ref) {
    return prefix + ":" + keyrefstring(raw_ref);
}

// Get the ID of the keyring we store all fscrypt keys in when the kernel is too
// old to support FS_IOC_ADD_ENCRYPTION_KEY and FS_IOC_REMOVE_ENCRYPTION_KEY.
static bool fscryptKeyring(key_serial_t* device_keyring) {
    *device_keyring = keyctl_search(KEY_SPEC_SESSION_KEYRING, "keyring", "fscrypt", 0);
    if (*device_keyring == -1) {
        PLOG(ERROR) << "Unable to find device keyring";
        return false;
    }
    return true;
}

// Add an encryption key to the legacy global session keyring.
static bool installKeyLegacy(const KeyBuffer& key, const std::string& raw_ref) {
    // Place fscrypt_key into automatically zeroing buffer.
    KeyBuffer fsKeyBuffer(sizeof(fscrypt_key));
    fscrypt_key& fs_key = *reinterpret_cast<fscrypt_key*>(fsKeyBuffer.data());

    if (!fillKey(key, &fs_key)) return false;
    key_serial_t device_keyring;
    if (!fscryptKeyring(&device_keyring)) return false;
    for (char const* const* name_prefix = NAME_PREFIXES; *name_prefix != nullptr; name_prefix++) {
        auto ref = buildLegacyKeyName(*name_prefix, raw_ref);
        key_serial_t key_id =
            add_key("logon", ref.c_str(), (void*)&fs_key, sizeof(fs_key), device_keyring);
        if (key_id == -1) {
            PLOG(ERROR) << "Failed to insert key into keyring " << device_keyring;
            return false;
        }
        LOG(DEBUG) << "Added key " << key_id << " (" << ref << ") to keyring " << device_keyring
                   << " in process " << getpid();
    }
    return true;
}

// Build a struct fscrypt_key_specifier for use in the key management ioctls.
static bool buildKeySpecifier(fscrypt_key_specifier* spec, const std::string& raw_ref,
                              int policy_version) {
    switch (policy_version) {
        case 1:
            if (raw_ref.size() != FSCRYPT_KEY_DESCRIPTOR_SIZE) {
                LOG(ERROR) << "Invalid key specifier size for v1 encryption policy: "
                           << raw_ref.size();
                return false;
            }
            spec->type = FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR;
            memcpy(spec->u.descriptor, raw_ref.c_str(), FSCRYPT_KEY_DESCRIPTOR_SIZE);
            return true;
        case 2:
            if (raw_ref.size() != FSCRYPT_KEY_IDENTIFIER_SIZE) {
                LOG(ERROR) << "Invalid key specifier size for v2 encryption policy: "
                           << raw_ref.size();
                return false;
            }
            spec->type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
            memcpy(spec->u.identifier, raw_ref.c_str(), FSCRYPT_KEY_IDENTIFIER_SIZE);
            return true;
        default:
            LOG(ERROR) << "Invalid encryption policy version: " << policy_version;
            return false;
    }
}

// Install a file-based encryption key to the kernel, for use by encrypted files
// on the specified filesystem using the specified encryption policy version.
//
// For v1 policies, we use FS_IOC_ADD_ENCRYPTION_KEY if the kernel supports it.
// Otherwise we add the key to the legacy global session keyring.
//
// For v2 policies, we always use FS_IOC_ADD_ENCRYPTION_KEY; it's the only way
// the kernel supports.
//
// Returns %true on success, %false on failure.  On success also sets *raw_ref
// to the raw key reference for use in the encryption policy.
bool installKey(const KeyBuffer& key, const std::string& mountpoint, int policy_version,
                std::string* raw_ref) {
    // Put the fscrypt_add_key_arg in an automatically-zeroing buffer, since we
    // have to copy the raw key into it.
    KeyBuffer arg_buf(sizeof(struct fscrypt_add_key_arg) + key.size(), 0);
    struct fscrypt_add_key_arg* arg = (struct fscrypt_add_key_arg*)arg_buf.data();

    // Initialize the "key specifier", which is like a name for the key.
    switch (policy_version) {
        case 1:
            // A key for a v1 policy is specified by an arbitrary 8-byte
            // "descriptor", which must be provided by userspace.  We use the
            // first 8 bytes from the double SHA-512 of the key itself.
            *raw_ref = generateKeyRef((const uint8_t*)key.data(), key.size());
            if (!isFsKeyringSupported()) {
                return installKeyLegacy(key, *raw_ref);
            }
            if (!buildKeySpecifier(&arg->key_spec, *raw_ref, policy_version)) {
                return false;
            }
            break;
        case 2:
            // A key for a v2 policy is specified by an 16-byte "identifier",
            // which is a cryptographic hash of the key itself which the kernel
            // computes and returns.  Any user-provided value is ignored; we
            // just need to set the specifier type to indicate that we're adding
            // this type of key.
            arg->key_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
            break;
        default:
            LOG(ERROR) << "Invalid encryption policy version: " << policy_version;
            return false;
    }

    // Provide the raw key.
    arg->raw_size = key.size();
    memcpy(arg->raw, key.data(), key.size());

    android::base::unique_fd fd(open(mountpoint.c_str(), O_RDONLY | O_DIRECTORY | O_CLOEXEC));
    if (fd == -1) {
        PLOG(ERROR) << "Failed to open " << mountpoint << " to install key";
        return false;
    }

    if (ioctl(fd, FS_IOC_ADD_ENCRYPTION_KEY, arg) != 0) {
        PLOG(ERROR) << "Failed to install fscrypt key to " << mountpoint;
        return false;
    }

    if (arg->key_spec.type == FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER) {
        // Retrieve the key identifier that the kernel computed.
        *raw_ref = std::string((char*)arg->key_spec.u.identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);
    }
    LOG(DEBUG) << "Installed fscrypt key with ref " << keyrefstring(*raw_ref) << " to "
               << mountpoint;
    return true;
}

// Remove an encryption key from the legacy global session keyring.
static bool evictKeyLegacy(const std::string& raw_ref) {
    key_serial_t device_keyring;
    if (!fscryptKeyring(&device_keyring)) return false;
    bool success = true;
    for (char const* const* name_prefix = NAME_PREFIXES; *name_prefix != nullptr; name_prefix++) {
        auto ref = buildLegacyKeyName(*name_prefix, raw_ref);
        auto key_serial = keyctl_search(device_keyring, "logon", ref.c_str(), 0);

        // Unlink the key from the keyring.  Prefer unlinking to revoking or
        // invalidating, since unlinking is actually no less secure currently, and
        // it avoids bugs in certain kernel versions where the keyring key is
        // referenced from places it shouldn't be.
        if (keyctl_unlink(key_serial, device_keyring) != 0) {
            PLOG(ERROR) << "Failed to unlink key with serial " << key_serial << " ref " << ref;
            success = false;
        } else {
            LOG(DEBUG) << "Unlinked key with serial " << key_serial << " ref " << ref;
        }
    }
    return success;
}

// Evict a file-based encryption key from the kernel.
//
// We use FS_IOC_REMOVE_ENCRYPTION_KEY if the kernel supports it.  Otherwise we
// remove the key from the legacy global session keyring.
//
// In the latter case, the caller is responsible for dropping caches.
bool evictKey(const std::string& mountpoint, const std::string& raw_ref, int policy_version) {
    if (policy_version == 1 && !isFsKeyringSupported()) {
        return evictKeyLegacy(raw_ref);
    }

    android::base::unique_fd fd(open(mountpoint.c_str(), O_RDONLY | O_DIRECTORY | O_CLOEXEC));
    if (fd == -1) {
        PLOG(ERROR) << "Failed to open " << mountpoint << " to evict key";
        return false;
    }

    struct fscrypt_remove_key_arg arg;
    memset(&arg, 0, sizeof(arg));

    if (!buildKeySpecifier(&arg.key_spec, raw_ref, policy_version)) {
        return false;
    }

    std::string ref = keyrefstring(raw_ref);

    if (ioctl(fd, FS_IOC_REMOVE_ENCRYPTION_KEY, &arg) != 0) {
        PLOG(ERROR) << "Failed to evict fscrypt key with ref " << ref << " from " << mountpoint;
        return false;
    }

    LOG(DEBUG) << "Evicted fscrypt key with ref " << ref << " from " << mountpoint;
    if (arg.removal_status_flags & FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS) {
        // Should never happen because keys are only added/removed as root.
        LOG(ERROR) << "Unexpected case: key with ref " << ref << " is still added by other users!";
    } else if (arg.removal_status_flags & FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY) {
        LOG(ERROR) << "Files still open after removing key with ref " << ref
                   << ".  These files were not locked!";
    }
    return true;
}

bool retrieveAndInstallKey(bool create_if_absent, const KeyAuthentication& key_authentication,
                           const std::string& key_path, const std::string& tmp_path,
                           const std::string& volume_uuid, int policy_version,
                           std::string* key_ref) {
    KeyBuffer key;
    if (pathExists(key_path)) {
        LOG(DEBUG) << "Key exists, using: " << key_path;
        if (!retrieveKey(key_path, key_authentication, &key)) return false;
    } else {
        if (!create_if_absent) {
            LOG(ERROR) << "No key found in " << key_path;
            return false;
        }
        LOG(INFO) << "Creating new key in " << key_path;
        if (!randomKey(&key)) return false;
        if (!storeKeyAtomically(key_path, tmp_path, key_authentication, key)) return false;
    }

    if (!installKey(key, BuildDataPath(volume_uuid), policy_version, key_ref)) {
        LOG(ERROR) << "Failed to install key in " << key_path;
        return false;
    }
    return true;
}

bool retrieveKey(bool create_if_absent, const std::string& key_path, const std::string& tmp_path,
                 KeyBuffer* key, bool keepOld) {
    if (pathExists(key_path)) {
        LOG(DEBUG) << "Key exists, using: " << key_path;
        if (!retrieveKey(key_path, kEmptyAuthentication, key, keepOld)) return false;
    } else {
        if (!create_if_absent) {
            LOG(ERROR) << "No key found in " << key_path;
            return false;
        }
        LOG(INFO) << "Creating new key in " << key_path;
        if (!randomKey(key)) return false;
        if (!storeKeyAtomically(key_path, tmp_path, kEmptyAuthentication, *key)) return false;
    }
    return true;
}

}  // namespace vold
}  // namespace android