creds-util: permit credentials encrypted/signed by fixed zero length keys as fallback for systems lacking TPM2

This is supposed to be useful when generating credentials for immutable
initrd environments, where it is is relevant to support credentials even
on systems lacking a TPM2 chip.

With this, if `systemd-creds encrypt --with-key=auto-initrd` is used a
credential will be encrypted/signed with the TPM2 if it is available and
recognized by the firmware. Otherwise it will be encrypted/signed with
the fixed empty key, thus providing no confidentiality or authenticity.

The idea is that distributions use this mode to generically create
credentials that are as locked down as possible on the specific
platform.

1 files changed, 11 insertions, 4 deletions

diff --git a/src/shared/creds-util.h b/src/shared/creds-util.h
index 7f0ce421ad..5e4b48220d 100644
--- a/src/shared/creds-util.h
+++ b/src/shared/creds-util.h
@@ -38,15 +38,22 @@ typedef enum CredentialSecretFlags {
 
 int get_credential_host_secret(CredentialSecretFlags flags, void **ret, size_t *ret_size);
 
-/* The three modes we support: keyed only by on-disk key, only by TPM2 HMAC key, and by the combination of both */
+/* The four modes we support: keyed only by on-disk key, only by TPM2 HMAC key, and by the combination of
+ * both, as well as one with a fixed zero length key if TPM2 is missing (the latter of course provides no
+ * authenticity or confidentiality, but is still useful for integrity protection, and makes things simpler
+ * for us to handle). */
 #define CRED_AES256_GCM_BY_HOST               SD_ID128_MAKE(5a,1c,6a,86,df,9d,40,96,b1,d5,a6,5e,08,62,f1,9a)
 #define CRED_AES256_GCM_BY_TPM2_HMAC          SD_ID128_MAKE(0c,7c,c0,7b,11,76,45,91,9c,4b,0b,ea,08,bc,20,fe)
 #define CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC SD_ID128_MAKE(93,a8,94,09,48,74,44,90,90,ca,f2,fc,93,ca,b5,53)
+#define CRED_AES256_GCM_BY_TPM2_ABSENT        SD_ID128_MAKE(05,84,69,da,f6,f5,43,24,80,05,49,da,0f,8e,a2,fb)
 
-/* Special ID to pick automatic mode (i.e. tpm2+host if TPM2 exists, only host otherwise). This ID will never
- * be stored on disk, but is useful only internally while figuring out what precisely to write to disk. To
- * mark that this isn't a "real" type, we'll prefix it with an underscore. */
+/* Two special IDs to pick a general automatic mode (i.e. tpm2+host if TPM2 exists, only host otherwise) or
+ * an initrd-specific automatic mode (i.e. tpm2 if firmware can do it, otherwise fixed zero-length key, and
+ * never involve host keys). These IDs will never be stored on disk, but are useful only internally while
+ * figuring out what precisely to write to disk. To mark that these aren't a "real" type, we'll prefix them
+ * with an underscore. */
 #define _CRED_AUTO                            SD_ID128_MAKE(a2,19,cb,07,85,b2,4c,04,b1,6d,18,ca,b9,d2,ee,01)
+#define _CRED_AUTO_INITRD                     SD_ID128_MAKE(02,dc,8e,de,3a,02,43,ab,a9,ec,54,9c,05,e6,a0,71)
 
 int encrypt_credential_and_warn(sd_id128_t with_key, const char *name, usec_t timestamp, usec_t not_after, const char *tpm2_device, uint32_t tpm2_pcr_mask, const void *input, size_t input_size, void **ret, size_t *ret_size);
 int decrypt_credential_and_warn(const char *validate_name, usec_t validate_timestamp, const char *tpm2_device, const void *input, size_t input_size, void **ret, size_t *ret_size);