[PATCH 2/2] tpm: st33zp24: Validate locality read result

[Ruoyu Wang <ruoyuw560@gmail.com>]

check_locality() treats every nonzero transport return as success. SPI
errors remain negative, while the I2C path can convert a negative write
error through its byte-sized status variable. Either result is nonzero
even though the TPM_ACCESS byte can remain unwritten, so indeterminate
ACTIVE_LOCALITY and VALID bits can falsely report an active locality.

Require recv() to return exactly the requested byte before examining
TPM_ACCESS. Transport errors and short reads now report an inactive
locality, while successful reads retain the existing behavior.

This issue was found by a static analysis checker and confirmed by manual
source review.

Fixes: 251a7b08213a ("TPM: STMicroelectronics ST33 I2C KERNEL 3.x")
Signed-off-by: Ruoyu Wang <ruoyuw560@gmail.com>
---
 drivers/char/tpm/st33zp24/st33zp24.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tpm/st33zp24/st33zp24.c b/drivers/char/tpm/st33zp24/st33zp24.c
index 898e8d01d26698..0e2deff94c3672 100644
--- a/drivers/char/tpm/st33zp24/st33zp24.c
+++ b/drivers/char/tpm/st33zp24/st33zp24.c
@@ -106,10 +106,10 @@ static bool check_locality(struct tpm_chip *chip)
 {
 	struct st33zp24_dev *tpm_dev = dev_get_drvdata(&chip->dev);
 	u8 data;
-	u8 status;
+	int status;
 
 	status = tpm_dev->ops->recv(tpm_dev->phy_id, TPM_ACCESS, &data, 1);
-	if (status && (data &
+	if (status == 1 && (data &
 		(TPM_ACCESS_ACTIVE_LOCALITY | TPM_ACCESS_VALID)) ==
 		(TPM_ACCESS_ACTIVE_LOCALITY | TPM_ACCESS_VALID))
 		return true;
-- 
2.51.0