cryptenroll: do not implicitly verify with default tpm policy signature

If it was not requested to use a tpm2 signature file when enrolling, do
not fallback to the default /run/systemd/tpm2-pcr-signature.json as it
likely will be unrelated if it exists.

Fixes https://github.com/systemd/systemd/issues/25435

src/cryptenroll/cryptenroll-tpm2.c

@@ -196,8 +196,9 @@ int enroll_tpm2(struct crypt_device *cd,
 
                 log_debug_errno(r, "Failed to read TPM2 PCR public key, proceeding without: %m");
                 pubkey_pcr_mask = 0;
-        } else {
-                /* Also try to load the signature JSON object, to verify that our enrollment will work. This is optional however. */
+        } else if (signature_path) {
+                /* Also try to load the signature JSON object, to verify that our enrollment will work.
+                 * This is optional however, skip it if it's not explicitly provided. */
 
                 r = tpm2_load_pcr_signature(signature_path, &signature_json);
                 if (r < 0) {

test/units/testsuite-70.sh

@@ -128,6 +128,12 @@ if [ -e /usr/lib/systemd/systemd-measure ] && \
     # Now, do the same, but with a cryptsetup binding
     truncate -s 20M $img
     cryptsetup luksFormat -q --pbkdf pbkdf2 --pbkdf-force-iterations 1000 --use-urandom $img /tmp/passphrase
+    # Ensure that an unrelated signature, when not requested, is not used
+    touch /run/systemd/tpm2-pcr-signature.json
+    systemd-cryptenroll --unlock-key-file=/tmp/passphrase --tpm2-device=auto --tpm2-public-key="/tmp/pcrsign-public.pem" $img
+    # Reset and use the signature now
+    rm -f /run/systemd/tpm2-pcr-signature.json
+    systemd-cryptenroll --wipe-slot=tpm2 $img
     systemd-cryptenroll --unlock-key-file=/tmp/passphrase --tpm2-device=auto --tpm2-public-key="/tmp/pcrsign-public.pem" --tpm2-signature="/tmp/pcrsign.sig2" $img
 
     # Check if we can activate that (without the token module stuff)