[PATCH 6.18.y] tpm: fix off-by-four bounds check in tpm2_get_random()

[Jarkko Sakkinen <jarkko@kernel.org>]

From: Jiangshan Yi <yijiangshan@kylinos.cn>

[ Upstream commit 9b522400bf5c082feb9a0037a053ea822a2c7e4d ]

When the response carries the TPM2_ST_SESSIONS tag, tpm2_get_random()
skips the 4-byte parameter size field before locating the random data,
but the bounds check still validates the response length against
TPM_HEADER_SIZE.  A truncated response can pass the check and make
memcpy() read up to 4 bytes past the response end, so stale buffer
contents end up in the caller's random bytes.

Fix this by checking the response length against 'offset', which
already includes the skipped parameter size field.

Cc: stable@vger.kernel.org #  v6.12+
Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260902074839.417419-1-yijiangshan%40kylinos.cn
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20260903035837.219284-1-yijiangshan@kylinos.cn
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
 drivers/char/tpm/tpm2-cmd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index 2f506cdb5f45..c5465834396a 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -298,7 +298,7 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 		out = (struct tpm2_get_random_out *)&buf.data[offset];
 		recd = min_t(u32, be16_to_cpu(out->size), num_bytes);
 		if (tpm_buf_length(&buf) <
-		    TPM_HEADER_SIZE +
+		    offset +
 		    offsetof(struct tpm2_get_random_out, buffer) +
 		    recd) {
 			err = -EFAULT;
-- 
2.47.3