[PATCH 1/2] tpm: st33zp24: Return zero on status read failure

[Ruoyu Wang <ruoyuw560@gmail.com>]

st33zp24_status() ignores the result of the transport read and returns
data even when no byte was received. The I2C transport, for example,
skips i2c_master_recv() when the register-select write is short or fails,
leaving data uninitialized. The resulting stack value can be interpreted
as TPM_STS flags and let status checks complete spuriously.

The status callback cannot propagate a transport error. Return zero
unless recv() reports exactly one byte. With no status bits set, callers
retry or take their existing timeout or error path instead of acting on
an invalid status value.

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, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/st33zp24/st33zp24.c b/drivers/char/tpm/st33zp24/st33zp24.c
index e2b7451ea7ccd3..898e8d01d26698 100644
--- a/drivers/char/tpm/st33zp24/st33zp24.c
+++ b/drivers/char/tpm/st33zp24/st33zp24.c
@@ -93,7 +93,9 @@ static u8 st33zp24_status(struct tpm_chip *chip)
 	struct st33zp24_dev *tpm_dev = dev_get_drvdata(&chip->dev);
 	u8 data;
 
-	tpm_dev->ops->recv(tpm_dev->phy_id, TPM_STS, &data, 1);
+	if (tpm_dev->ops->recv(tpm_dev->phy_id, TPM_STS, &data, 1) != 1)
+		return 0;
+
 	return data;
 }
 
-- 
2.51.0