IMA: Define a new template field buf

A buffer(kexec boot command line arguments) measured into IMA
measuremnt list cannot be appraised, without already being
aware of the buffer contents. Since hashes are non-reversible,
raw buffer is needed for validation or regenerating hash for
appraisal/attestation.

Add support to store/read the buffer contents in HEX.
The kexec cmdline hash is stored in the "d-ng" field of the
template data.  It can be verified using
sudo cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements |
  grep  kexec-cmdline | cut -d' ' -f 6 | xxd -r -p | sha256sum

- Add two new fields to ima_event_data to hold the buf and
buf_len
- Add a new template field 'buf' to be used to store/read
the buffer data.
- Updated process_buffer_meaurement to add the buffer to
ima_event_data. process_buffer_measurement added in
"Define a new IMA hook to measure the boot command line
 arguments"
- Add a new template policy name ima-buf to represent
'd-ng|n-ng|buf'

Signed-off-by: Prakhar Srivastava <prsriva02@gmail.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_template.c

@@ -26,6 +26,7 @@ static struct ima_template_desc builtin_templates[] = {
 	{.name = IMA_TEMPLATE_IMA_NAME, .fmt = IMA_TEMPLATE_IMA_FMT},
 	{.name = "ima-ng", .fmt = "d-ng|n-ng"},
 	{.name = "ima-sig", .fmt = "d-ng|n-ng|sig"},
+	{.name = "ima-buf", .fmt = "d-ng|n-ng|buf"},
 	{.name = "", .fmt = ""},	/* placeholder for a custom format */
 };
 
@@ -43,6 +44,8 @@ static const struct ima_template_field supported_fields[] = {
 	 .field_show = ima_show_template_string},
 	{.field_id = "sig", .field_init = ima_eventsig_init,
 	 .field_show = ima_show_template_sig},
+	{.field_id = "buf", .field_init = ima_eventbuf_init,
+	 .field_show = ima_show_template_buf},
 };
 #define MAX_TEMPLATE_NAME_LEN 15