tpm2_key_protector: Support PCR capping

To prevent a sealed key from being unsealed again, a common and
straightforward method is to "cap" the key by extending the associated
PCRs. When the PCRs associated with the sealed key are extended, TPM will
be unable to unseal the key, as the PCR values required for unsealing no
longer match, effectively rendering the key unusable until the next
system boot or a state where the PCRs are reset to their expected values.

To cap a specific set of PCRs, simply append the argument '-c pcr_list'
to the tpm2_key_protector command. Upon successfully unsealing the key,
the TPM2 key protector will then invoke tpm2_protector_cap_pcrs(). This
function extends the selected PCRs with an EV_SEPARATOR event,
effectively "capping" them. Consequently, the associated key cannot be
unsealed in any subsequent attempts until these PCRs are reset to their
original, pre-capped state, typically occurring upon the next system
boot.

Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/tpm2_key_protector/module.c

@@ -28,6 +28,7 @@
 #include <tss2_buffer.h>
 #include <tss2_types.h>
 #include <tss2_mu.h>
+#include <tcg2.h>
 
 #include "tpm2_args.h"
 #include "tpm2.h"
@@ -47,6 +48,7 @@ typedef enum tpm2_protector_options
   OPTION_MODE,
   OPTION_PCRS,
   OPTION_BANK,
+  OPTION_CAPPCRS,
   OPTION_TPM2KEY,
   OPTION_KEYFILE,
   OPTION_SRK,
@@ -61,6 +63,8 @@ typedef struct tpm2_protector_context
   grub_uint8_t pcr_count;
   grub_srk_type_t srk_type;
   TPM_ALG_ID_t bank;
+  grub_uint8_t cap_pcrs[TPM_MAX_PCRS];
+  grub_uint8_t cap_pcr_count;
   const char *tpm2key;
   const char *keyfile;
   TPM_HANDLE_t srk;
@@ -100,6 +104,16 @@ static const struct grub_arg_option tpm2_protector_init_cmd_options[] =
 	N_("Bank of PCRs used to authorize key release: "
 	   "SHA1, SHA256, SHA384 or SHA512. (default: SHA256)"),
     },
+    {
+      .longarg  = "cap-pcrs",
+      .shortarg = 'c',
+      .flags    = 0,
+      .arg      = NULL,
+      .type     = ARG_TYPE_STRING,
+      .doc      =
+	N_("Comma-separated list of PCRs to be capped after key release "
+	   "e.g., '7,11'."),
+    },
     /* SRK-mode options */
     {
       .longarg  = "tpm2key",
@@ -1212,19 +1226,45 @@ tpm2_protector_nv_recover (const tpm2_protector_context_t *ctx,
   return err;
 }
 
+static grub_err_t
+tpm2_protector_cap_pcrs (const tpm2_protector_context_t *ctx)
+{
+  grub_uint8_t i;
+  grub_err_t err;
+
+  for (i = 0; i < ctx->cap_pcr_count; i++)
+    {
+	err = grub_tcg2_cap_pcr (ctx->cap_pcrs[i]);
+	if (err != GRUB_ERR_NONE)
+	  return err;
+    }
+
+  return GRUB_ERR_NONE;
+}
+
 static grub_err_t
 tpm2_protector_recover (const tpm2_protector_context_t *ctx,
 			grub_uint8_t **key, grub_size_t *key_size)
 {
+  grub_err_t err;
+
   switch (ctx->mode)
     {
     case TPM2_PROTECTOR_MODE_SRK:
-      return tpm2_protector_srk_recover (ctx, key, key_size);
+      err = tpm2_protector_srk_recover (ctx, key, key_size);
+      break;
     case TPM2_PROTECTOR_MODE_NV:
-      return tpm2_protector_nv_recover (ctx, key, key_size);
+      err = tpm2_protector_nv_recover (ctx, key, key_size);
+      break;
     default:
-      return GRUB_ERR_BAD_ARGUMENT;
+      err = grub_error (GRUB_ERR_BAD_ARGUMENT, N_("Unknown Mode"));
     }
+
+  /* Cap the selected PCRs when the key is unsealed successfully */
+  if (ctx->cap_pcr_count > 0 && err == GRUB_ERR_NONE)
+    err = tpm2_protector_cap_pcrs (ctx);
+
+  return err;
 }
 
 static grub_err_t
@@ -1364,6 +1404,15 @@ tpm2_protector_init_cmd_handler (grub_extcmd_context_t ctxt, int argc,
 	return err;
     }
 
+  if (state[OPTION_CAPPCRS].set)  /* cap-pcrs */
+    {
+      err = grub_tpm2_protector_parse_pcrs (state[OPTION_CAPPCRS].arg,
+					    tpm2_protector_ctx.cap_pcrs,
+					    &tpm2_protector_ctx.cap_pcr_count);
+      if (err != GRUB_ERR_NONE)
+	return err;
+    }
+
   if (state[OPTION_TPM2KEY].set)  /* tpm2key */
     {
       err = tpm2_protector_parse_file (state[OPTION_TPM2KEY].arg,
@@ -1465,6 +1514,7 @@ GRUB_MOD_INIT (tpm2_key_protector)
 			  N_("[-m mode] "
 			     "[-p pcr_list] "
 			     "[-b pcr_bank] "
+			     "[-c pcr_list] "
 			     "[-T tpm2_key_file_path] "
 			     "[-k sealed_key_file_path] "
 			     "[-s srk_handle] "