tpm2_nv(read|write): use a default buffer size if TPM reports 0.

On a NUC5i7RYH, the TPM reports it's max NV buffer size as 0, which
means we can neither read nor write. In this case, default to
512 as that is generally considered a safe size.

Signed-off-by: William Roberts <william.c.roberts@intel.com>

lib/tpm2_nv_util.h

@@ -36,6 +36,11 @@
 #include "log.h"
 #include "tpm2_util.h"
 
+/*
+ * The default buffer size when one cannot be determined via get capability.
+ */
+#define NV_DEFAULT_BUFFER_SIZE 512
+
 /**
  * Reads the public portion of a Non-Volatile (nv) index.
  * @param sapi_context

tools/tpm2_nvread.c

@@ -123,6 +123,9 @@ static bool nv_read(TSS2_SYS_CONTEXT *sapi_context, tpm2_option_flags flags) {
     if (max_data_size > MAX_NV_BUFFER_SIZE) {
         max_data_size = MAX_NV_BUFFER_SIZE;
     }
+    else if (max_data_size == 0) {
+        max_data_size = NV_DEFAULT_BUFFER_SIZE;
+    }
 
     UINT8 *data_buffer = malloc(data_size);
     if (!data_buffer) {

tools/tpm2_nvwrite.c

@@ -119,6 +119,9 @@ static bool nv_write(TSS2_SYS_CONTEXT *sapi_context) {
     if (max_data_size > MAX_NV_BUFFER_SIZE) {
         max_data_size = MAX_NV_BUFFER_SIZE;
     }
+    else if (max_data_size == 0) {
+        max_data_size = NV_DEFAULT_BUFFER_SIZE;
+    }
 
     UINT16 data_offset = 0;
     UINT16 bytes_left = ctx.nv_buffer.t.size;