tpm2-util: refuse tokens with public key policy reference but no key

tpm2_parse_luks2_json() only insists on "tpm2_pubkey" being present when
"tpm2_pubkey_pcrs" is set, but happily accepts a token carrying
"tpm2_pubkey_ref" without a public key. tpm2_unseal() then trips over its
assert(!pubkey_policy_ref || iovec_is_set(pubkey)), i.e. a crafted LUKS2
header aborts whatever tries to unlock it instead of being rejected.

tpm2_make_luks2_json() never writes that combination (it asserts the same
invariant), hence refuse it when parsing, as is already done for a PCR
mask without a public key. This matters more now that systemd-dissect,
systemd-nspawn and systemd-sysext unlock images via these tokens, as the
headers they see are less trusted than the host's own volumes.

(cherry picked from commit 3520b44f7e398f924c267ee4527941924297c6bc)

src/shared/tpm2-util.c

@@ -11664,6 +11664,8 @@ int tpm2_parse_luks2_json(
                 const char *s = sd_json_variant_string(w);
                 if (!s)
                         return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "Public key policy reference is not a string.");
+                if (!iovec_is_set(&pubkey))
+                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "Public key policy reference set, but no public key included in JSON data, refusing.");
                 pubkey_ref = strdup(s);
                 if (!pubkey_ref)
                         return log_oom_debug();