cryptfs: Optionally get crypto type as a property
Instead of hardcoding to "aes-cbc-essiv:sha256" with a 16 byte
key, we introduce a new property, "ro.crypto.fde_algorithm",
to allow the use of different crypto types. The only other
method we currently support is "speck128-xts-plain64" with
a 32 byte key, although new crypto types are easily added.
We intentionally derive things like the crypto name and the
keysize from the given property name. This means the code
must be changed for each new crypto type we want to support,
but that's worth it to remove the exploit vector of crypto
types with incorrect key sizes.
Due to previous refactoring CLs, this has minimal impact on
the current code other than changing what we return for
cryptfs_get_{keysize,crypto_name}.
Bug: 73079191
Test: Flashed onto a gobo device with the property set for SPECK, and confirmed via kernel debug output we were using SPECK on the device.
Change-Id: I9c9df61590344c5f62114dfbf679031b0c2ceb1f
diff --git a/cryptfs.cpp b/cryptfs.cpp
index 62eb9a7..09e67af 100644
--- a/cryptfs.cpp
+++ b/cryptfs.cpp
@@ -72,7 +72,6 @@
#define DM_CRYPT_BUF_SIZE 4096
#define HASH_COUNT 2000
-#define DEFAULT_KEY_LEN_BYTES 16
constexpr size_t INTERMEDIATE_KEY_LEN_BYTES = 16;
constexpr size_t INTERMEDIATE_IV_LEN_BYTES = 16;
@@ -263,6 +262,122 @@
}
}
+namespace {
+
+struct CryptoType;
+
+// Use to get the CryptoType in use on this device.
+const CryptoType &get_crypto_type();
+
+struct CryptoType {
+ // We should only be constructing CryptoTypes as part of
+ // supported_crypto_types[]. We do it via this pseudo-builder pattern,
+ // which isn't pure or fully protected as a concession to being able to
+ // do it all at compile time. Add new CryptoTypes in
+ // supported_crypto_types[] below.
+ constexpr CryptoType() : CryptoType(nullptr, nullptr, 0xFFFFFFFF) {}
+ constexpr CryptoType set_keysize(uint32_t size) const {
+ return CryptoType(this->property_name, this->crypto_name, size);
+ }
+ constexpr CryptoType set_property_name(const char *property) const {
+ return CryptoType(property, this->crypto_name, this->keysize);
+ }
+ constexpr CryptoType set_crypto_name(const char *crypto) const {
+ return CryptoType(this->property_name, crypto, this->keysize);
+ }
+
+ constexpr const char *get_property_name() const { return property_name; }
+ constexpr const char *get_crypto_name() const { return crypto_name; }
+ constexpr uint32_t get_keysize() const { return keysize; }
+
+ private:
+ const char *property_name;
+ const char *crypto_name;
+ uint32_t keysize;
+
+ constexpr CryptoType(const char *property, const char *crypto,
+ uint32_t ksize)
+ : property_name(property), crypto_name(crypto), keysize(ksize) {}
+ friend const CryptoType &get_crypto_type();
+ static const CryptoType &get_device_crypto_algorithm();
+};
+
+// We only want to parse this read-only property once. But we need to wait
+// until the system is initialized before we can read it. So we use a static
+// scoped within this function to get it only once.
+const CryptoType &get_crypto_type() {
+ static CryptoType crypto_type = CryptoType::get_device_crypto_algorithm();
+ return crypto_type;
+}
+
+constexpr CryptoType default_crypto_type = CryptoType()
+ .set_property_name("AES-128-CBC")
+ .set_crypto_name("aes-cbc-essiv:sha256")
+ .set_keysize(16);
+
+constexpr CryptoType supported_crypto_types[] = {
+ default_crypto_type,
+ CryptoType()
+ .set_property_name("Speck128/128-XTS")
+ .set_crypto_name("speck128-xts-plain64")
+ .set_keysize(32),
+ // Add new CryptoTypes here. Order is not important.
+};
+
+
+// ---------- START COMPILE-TIME SANITY CHECK BLOCK -------------------------
+// We confirm all supported_crypto_types have a small enough keysize and
+// had both set_property_name() and set_crypto_name() called.
+
+template <typename T, size_t N>
+constexpr size_t array_length(T (&)[N]) { return N; }
+
+constexpr bool indexOutOfBoundsForCryptoTypes(size_t index) {
+ return (index >= array_length(supported_crypto_types));
+}
+
+constexpr bool isValidCryptoType(const CryptoType &crypto_type) {
+ return ((crypto_type.get_property_name() != nullptr) &&
+ (crypto_type.get_crypto_name() != nullptr) &&
+ (crypto_type.get_keysize() <= MAX_KEY_LEN));
+}
+
+// Note in C++11 that constexpr functions can only have a single line.
+// So our code is a bit convoluted (using recursion instead of a loop),
+// but it's asserting at compile time that all of our key lengths are valid.
+constexpr bool validateSupportedCryptoTypes(size_t index) {
+ return indexOutOfBoundsForCryptoTypes(index) ||
+ (isValidCryptoType(supported_crypto_types[index]) &&
+ validateSupportedCryptoTypes(index + 1));
+}
+
+static_assert(validateSupportedCryptoTypes(0),
+ "We have a CryptoType with keysize > MAX_KEY_LEN or which was "
+ "incompletely constructed.");
+// ---------- END COMPILE-TIME SANITY CHECK BLOCK -------------------------
+
+
+// Don't call this directly, use get_crypto_type(), which caches this result.
+const CryptoType &CryptoType::get_device_crypto_algorithm() {
+ constexpr char CRYPT_ALGO_PROP[] = "ro.crypto.fde_algorithm";
+ char paramstr[PROPERTY_VALUE_MAX];
+
+ property_get(CRYPT_ALGO_PROP, paramstr,
+ default_crypto_type.get_property_name());
+ for (auto const &ctype : supported_crypto_types) {
+ if (strcmp(paramstr, ctype.get_property_name()) == 0) {
+ return ctype;
+ }
+ }
+ ALOGE("Invalid name (%s) for %s. Defaulting to %s\n", paramstr,
+ CRYPT_ALGO_PROP, default_crypto_type.get_property_name());
+ return default_crypto_type;
+}
+
+} // namespace
+
+
+
/**
* Gets the default device scrypt parameters for key derivation time tuning.
* The parameters should lead to about one second derivation time for the
@@ -283,11 +398,11 @@
}
uint32_t cryptfs_get_keysize() {
- return DEFAULT_KEY_LEN_BYTES;
+ return get_crypto_type().get_keysize();
}
const char *cryptfs_get_crypto_name() {
- return "aes-cbc-essiv:sha256";
+ return get_crypto_type().get_crypto_name();
}
static unsigned int get_fs_size(char *dev)