[PATCH v2] tpm_tis: Check return values from get_burstcount.

[Josh Zimmerman <joshz@google.com>]

If the TPM we're connecting to uses a static burst count, it will report
a burst count of zero throughout the response read. However, get_burstcount
assumes that a response of zero indicates that the TPM is not ready to
receive more data. In this case, it returns a negative error code, which
is passed on to tpm_tis_{write,read}_bytes as a u16, causing
them to read/write far too many bytes.

This patch checks for negative return codes and bails out from recv_data
and tpm_tis_send_data.

Fixes: 1107d065fdf1
---
 drivers/char/tpm/tpm_tis_core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
index e3bf31b..aed92b3 100644
--- a/drivers/char/tpm/tpm_tis_core.c
+++ b/drivers/char/tpm/tpm_tis_core.c
@@ -186,6 +186,12 @@ static int recv_data(struct tpm_chip *chip, u8 *buf, size_t count)
 				 chip->timeout_c,
 				 &priv->read_queue, true) == 0) {
 		burstcnt = min_t(int, get_burstcount(chip), count - size);
+		if (burstcnt < 0) {
+			dev_err(&chip->dev,
+				"Unable to read burstcount in %s:%d (%s)\n",
+				__FILE__, __LINE__, __func__);
+			return burstcnt;
+		}
 
 		rc = tpm_tis_read_bytes(priv, TPM_DATA_FIFO(priv->locality),
 					burstcnt, buf + size);
@@ -272,6 +278,13 @@ static int tpm_tis_send_data(struct tpm_chip *chip, u8 *buf, size_t len)
 
 	while (count < len - 1) {
 		burstcnt = min_t(int, get_burstcount(chip), len - count - 1);
+		if (burstcnt < 0) {
+			dev_err(&chip->dev,
+				"Unable to read burstcount in %s:%d (%s)\n",
+				__FILE__, __LINE__, __func__);
+			rc = burstcnt;
+			goto out_err;
+		}
 		rc = tpm_tis_write_bytes(priv, TPM_DATA_FIFO(priv->locality),
 					 burstcnt, buf + count);
 		if (rc < 0)
-- 
2.8.0.rc3.226.g39d4020