stubdom/grub: verify vTPM label if requested

This adds an optional argument --vtpm-label=<label> to the pv-grub
command line.  If specified, a vtpm device must be connected to the
pv-grub domain and the backend of this device must have the given XSM
label (which may start with a * to indicate a wildcard).  Verifying the
label of the vTPM before sending measurements prevents a disaggregated
control domain that has access to xenstore but not to the guest domains
from causing the measurements performed by pv-grub to be discarded,
allowing the forgery of arbitrary kernel measurements in the TPM.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

stubdom/grub/kexec.c

@@ -68,6 +68,14 @@ struct pcr_extend_cmd {
 	unsigned char hash[20];
 } __attribute__((packed));
 
+struct pcr_extend_rsp {
+	uint16_t tag;
+	uint32_t size;
+	uint32_t status;
+
+	unsigned char hash[20];
+} __attribute__((packed));
+
 /* Not imported from polarssl's header since the prototype unhelpfully defines
  * the input as unsigned char, which causes pointer type mismatches */
 void sha1(const void *input, size_t ilen, unsigned char output[20]);
@@ -135,20 +143,49 @@ int kexec_allocate(struct xc_dom_image *dom, xen_vaddr_t up_to)
     return 0;
 }
 
+/* Filled from mini-os command line or left as NULL */
+char *vtpm_label;
+
 static void tpm_hash2pcr(struct xc_dom_image *dom, char *cmdline)
 {
 	struct tpmfront_dev* tpm = init_tpmfront(NULL);
-	uint8_t *resp;
+	struct pcr_extend_rsp *resp;
 	size_t resplen = 0;
 	struct pcr_extend_cmd cmd;
+	int rv;
 
-	/* If all guests have access to a vTPM, it may be useful to replace this
-	 * with ASSERT(tpm) to prevent configuration errors from allowing a guest
-	 * to boot without a TPM (or with a TPM that has not been sent any
-	 * measurements, which could allow forging the measurements).
+	/*
+	 * If vtpm_label was specified on the command line, require a vTPM to be
+	 * attached and for the domain providing the vTPM to have the given
+	 * label.
 	 */
-	if (!tpm)
+	if (vtpm_label) {
+		char ctx[128];
+		if (!tpm) {
+			printf("No TPM found and vtpm_label specified, aborting!\n");
+			do_exit();
+		}
+		rv = evtchn_get_peercontext(tpm->evtchn, ctx, sizeof(ctx) - 1);
+		if (rv < 0) {
+			printf("Could not verify vtpm_label: %d\n", rv);
+			do_exit();
+		}
+		ctx[127] = 0;
+		rv = strcmp(ctx, vtpm_label);
+		if (rv && vtpm_label[0] == '*') {
+			int match_len = strlen(vtpm_label) - 1;
+			int offset = strlen(ctx) - match_len;
+			if (offset > 0)
+				rv = strcmp(ctx + offset, vtpm_label + 1);
+		}
+
+		if (rv) {
+			printf("Mismatched vtpm_label: '%s' != '%s'\n", ctx, vtpm_label);
+			do_exit();
+		}
+	} else if (!tpm) {
 		return;
+	}
 
 	cmd.tag = bswap_16(TPM_TAG_RQU_COMMAND);
 	cmd.size = bswap_32(sizeof(cmd));
@@ -156,15 +193,18 @@ static void tpm_hash2pcr(struct xc_dom_image *dom, char *cmdline)
 	cmd.pcr = bswap_32(4); // PCR #4 for kernel
 	sha1(dom->kernel_blob, dom->kernel_size, cmd.hash);
 
-	tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), &resp, &resplen);
+	rv = tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), (void*)&resp, &resplen);
+	ASSERT(rv == 0 && resp->status == 0);
 
 	cmd.pcr = bswap_32(5); // PCR #5 for cmdline
 	sha1(cmdline, strlen(cmdline), cmd.hash);
-	tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), &resp, &resplen);
+	rv = tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), (void*)&resp, &resplen);
+	ASSERT(rv == 0 && resp->status == 0);
 
 	cmd.pcr = bswap_32(5); // PCR #5 for initrd
 	sha1(dom->ramdisk_blob, dom->ramdisk_size, cmd.hash);
-	tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), &resp, &resplen);
+	rv = tpmfront_cmd(tpm, (void*)&cmd, sizeof(cmd), (void*)&resp, &resplen);
+	ASSERT(rv == 0 && resp->status == 0);
 
 	shutdown_tpmfront(tpm);
 }

stubdom/grub/mini-os.c

@@ -735,8 +735,14 @@ void __attribute__ ((noreturn)) grub_reboot (void)
  * for grub's 32bit pointers to work */
 char grub_scratch_mem[SCRATCH_MEMSIZE] __attribute__((aligned(PAGE_SIZE)));
 
-int main(int argc, char *argv[])
+int main(int argc, char **argv)
 {
+    if (argc > 1 && memcmp(argv[1], "--vtpm-label=", 13) == 0) {
+        vtpm_label = argv[1] + 13;
+        argc--;
+        argv++;
+    }
+
     if (argc > 1) {
         strncpy(config_file, argv[1], sizeof(config_file) - 1);
         config_file[sizeof(config_file) - 1] = 0;

stubdom/grub/mini-os.h

@@ -3,3 +3,5 @@ extern struct blkfront_dev **blk_dev;
 extern struct netfront_dev *net_dev;
 extern struct kbdfront_dev *kbd_dev;
 extern struct fbfront_dev *fb_dev;
+
+extern char* vtpm_label;