tpm: tpm_passthrough: Fail startup if FE buffer size < BE buffer size

If the requested buffer size of the frontend is smaller than the fixed
buffer size of the host's TPM, fail the startup_tpm() interface function,
which will make the device unusable. We fail it because the backend TPM
could produce larger packets than what the frontend could pass to the OS.

The current combination of TIS frontend and either passthrough or emulator
backend will not lead to this case since the TIS can support any size of
buffer.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

hw/tpm/tpm_passthrough.c

@@ -304,6 +304,20 @@ static TPMBackend *tpm_passthrough_create(QemuOpts *opts)
     return TPM_BACKEND(obj);
 }
 
+static int tpm_passthrough_startup_tpm(TPMBackend *tb, size_t buffersize)
+{
+    TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
+
+    if (buffersize && buffersize < tpm_pt->tpm_buffersize) {
+        error_report("Requested buffer size of %zu is smaller than host TPM's "
+                     "fixed buffer size of %zu",
+                     buffersize, tpm_pt->tpm_buffersize);
+        return -1;
+    }
+
+    return 0;
+}
+
 static TpmTypeOptions *tpm_passthrough_get_tpm_options(TPMBackend *tb)
 {
     TpmTypeOptions *options = g_new0(TpmTypeOptions, 1);
@@ -362,6 +376,7 @@ static void tpm_passthrough_class_init(ObjectClass *klass, void *data)
     tbc->opts = tpm_passthrough_cmdline_opts;
     tbc->desc = "Passthrough TPM backend driver";
     tbc->create = tpm_passthrough_create;
+    tbc->startup_tpm = tpm_passthrough_startup_tpm;
     tbc->reset = tpm_passthrough_reset;
     tbc->cancel_cmd = tpm_passthrough_cancel_cmd;
     tbc->get_tpm_established_flag = tpm_passthrough_get_tpm_established_flag;