tpm-util: use trial session where appropriate

TPM2 knows two types of policy sessions: "real" ones and "trial" ones. The
latter allow you to calculate a policy hash without this enforcing any
policy, which the former do. Typically you want to use the "trial" ones
when enrolling, and you have to use the "real" ones for unlocking. So
far we used "real" ones for both cases. Which works fine – as long as
the policy put together matches the current reality (e.g. the PCR values
included in the policy are the ones currently in place in the TPM).

Let's switch to using trial sessions for enrolling. First of all this is
preparation for later work to implement further policy extensions (for
example, policies binding to literally specified PCR values, instead of
the once currently measured). But from my perspective more importantly
it actually is cleaner, as it communicates more clearly what we are
actually doing here.

No user-visible change in behaviour.

diff --git a/src/shared/tpm2-util.c b/src/shared/tpm2-util.c
index 7ee49bf8dfc1b625cf9ca124fb978167f8c97313..33703c4aa3bf908d1c0bbc620ff46c7c6d026f81 100644 (file)
--- a/src/shared/tpm2-util.c
+++ b/src/shared/tpm2-util.c
@@ -661,6 +661,7 @@ static int tpm2_make_pcr_session(
                 ESYS_CONTEXT *c,
                 ESYS_TR primary,
                 ESYS_TR parent_session,
+                TPM2_SE session_type,
                 uint32_t pcr_mask,
                 uint16_t pcr_bank, /* If UINT16_MAX, pick best bank automatically, otherwise specify bank explicitly. */
                 bool use_pin,
@@ -711,7 +712,7 @@ static int tpm2_make_pcr_session(
                         ESYS_TR_NONE,
                         ESYS_TR_NONE,
                         NULL,
-                        TPM2_SE_POLICY,
+                        session_type,
                         &symmetric,
                         TPM2_ALG_SHA256,
                         &session);
@@ -880,8 +881,17 @@ int tpm2_seal(
         if (r < 0)
                 goto finish;
 
-        r = tpm2_make_pcr_session(c.esys_context, primary, session, pcr_mask, UINT16_MAX, !!pin, NULL,
-                                  &policy_digest, &pcr_bank);
+        r = tpm2_make_pcr_session(
+                        c.esys_context,
+                        primary,
+                        session,
+                        TPM2_SE_TRIAL,
+                        pcr_mask,
+                        /* pcr_bank= */ UINT16_MAX,
+                        !!pin,
+                        /* ret_session= */ NULL,
+                        &policy_digest,
+                        &pcr_bank);
         if (r < 0)
                 goto finish;
 
@@ -1085,8 +1095,17 @@ int tpm2_unseal(
         if (r < 0)
                 goto finish;
 
-        r = tpm2_make_pcr_session(c.esys_context, primary, hmac_session, pcr_mask, pcr_bank, !!pin, &session,
-                                  &policy_digest, NULL);
+        r = tpm2_make_pcr_session(
+                        c.esys_context,
+                        primary,
+                        hmac_session,
+                        TPM2_SE_POLICY,
+                        pcr_mask,
+                        pcr_bank,
+                        !!pin,
+                        &session,
+                        &policy_digest,
+                        /* ret_pcr_bank= */ NULL);
         if (r < 0)
                 goto finish;