tpm: use tpm_buf functions in tpm2_pcr_read()

tpm2_pcr_read() now builds the PCR read command buffer with tpm_buf
functions. This solution is preferred to using a tpm2_cmd structure,
as tpm_buf functions provide protection against buffer overflow.

Signed-off-by: Roberto Sassu <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Tested-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

drivers/char/tpm/tpm2-cmd.c

···
 	u8	pcr_select[TPM2_PCR_SELECT_MIN];
 } __packed;
 
-struct tpm2_pcr_read_out {
-	__be32	update_cnt;
-	__be32	pcr_selects_cnt;
-	__be16	hash_alg;
-	u8	pcr_select_size;
-	u8	pcr_select[TPM2_PCR_SELECT_MIN];
-	__be32	digests_cnt;
-	__be16	digest_size;
-	u8	digest[TPM_DIGEST_SIZE];
-} __packed;
-
 struct tpm2_get_tpm_pt_in {
 	__be32	cap_id;
 	__be32	property_id;
···
 	struct	tpm2_startup_in		startup_in;
 	struct	tpm2_self_test_in	selftest_in;
 	struct	tpm2_pcr_read_in	pcrread_in;
-	struct	tpm2_pcr_read_out	pcrread_out;
 	struct	tpm2_get_tpm_pt_in	get_tpm_pt_in;
 	struct	tpm2_get_tpm_pt_out	get_tpm_pt_out;
 	struct	tpm2_get_random_in	getrandom_in;
···
 	(sizeof(struct tpm_input_header) + \
 	 sizeof(struct tpm2_pcr_read_in))
 
-#define TPM2_PCR_READ_RESP_BODY_SIZE \
-	 sizeof(struct tpm2_pcr_read_out)
-
 static const struct tpm_input_header tpm2_pcrread_header = {
 	.tag = cpu_to_be16(TPM2_ST_NO_SESSIONS),
 	.length = cpu_to_be32(TPM2_PCR_READ_IN_SIZE),
 	.ordinal = cpu_to_be32(TPM2_CC_PCR_READ)
 };
+
+struct tpm2_pcr_read_out {
+	__be32	update_cnt;
+	__be32	pcr_selects_cnt;
+	__be16	hash_alg;
+	u8	pcr_select_size;
+	u8	pcr_select[TPM2_PCR_SELECT_MIN];
+	__be32	digests_cnt;
+	__be16	digest_size;
+	u8	digest[];
+} __packed;
 
 /**
  * tpm2_pcr_read() - read a PCR value
···
 int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
 {
 	int rc;
-	struct tpm2_cmd cmd;
-	u8 *buf;
+	struct tpm_buf buf;
+	struct tpm2_pcr_read_out *out;
+	u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
 
 	if (pcr_idx >= TPM2_PLATFORM_PCR)
 		return -EINVAL;
 
-	cmd.header.in = tpm2_pcrread_header;
-	cmd.params.pcrread_in.pcr_selects_cnt = cpu_to_be32(1);
-	cmd.params.pcrread_in.hash_alg = cpu_to_be16(TPM2_ALG_SHA1);
-	cmd.params.pcrread_in.pcr_select_size = TPM2_PCR_SELECT_MIN;
+	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
+	if (rc)
+		return rc;
 
-	memset(cmd.params.pcrread_in.pcr_select, 0,
-	       sizeof(cmd.params.pcrread_in.pcr_select));
-	cmd.params.pcrread_in.pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
+	pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
 
-	rc = tpm_transmit_cmd(chip, NULL, &cmd, sizeof(cmd),
-			      TPM2_PCR_READ_RESP_BODY_SIZE,
-			      0, "attempting to read a pcr value");
-	if (rc == 0) {
-		buf = cmd.params.pcrread_out.digest;
-		memcpy(res_buf, buf, TPM_DIGEST_SIZE);
+	tpm_buf_append_u32(&buf, 1);
+	tpm_buf_append_u16(&buf, TPM2_ALG_SHA1);
+	tpm_buf_append_u8(&buf, TPM2_PCR_SELECT_MIN);
+	tpm_buf_append(&buf, (const unsigned char *)pcr_select,
+		       sizeof(pcr_select));
+
+	rc = tpm_transmit_cmd(chip, NULL, buf.data, PAGE_SIZE, 0, 0,
+			res_buf ? "attempting to read a pcr value" : NULL);
+	if (rc == 0 && res_buf) {
+		out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
+		memcpy(res_buf, out->digest, SHA1_DIGEST_SIZE);
 	}
 
+	tpm_buf_destroy(&buf);
 	return rc;
 }