[PATCH 6.12.y] tpm: fix off-by-four bounds check in tpm2_get_random()

[PATCH 6.12.y] tpm: fix off-by-four bounds check in tpm2_get_random()

[Jarkko Sakkinen]

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 c710128f49b1..63d26aebf55d 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -361,7 +361,7 @@ 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

Re: [PATCH 6.12.y] tpm: fix off-by-four bounds check in tpm2_get_random()

[Sasha Levin]

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

Queued for 6.12, thanks.

-- 
Thanks,
Sasha

Re: [PATCH 6.12.y] tpm: fix off-by-four bounds check in tpm2_get_random()

[Jarkko Sakkinen]

On Mon, Oct 05, 2026 at 09:50:27AM -0400, Sasha Levin wrote:
> > Fix this by checking the response length against 'offset', which
> > already includes the skipped parameter size field.
> 
> Queued for 6.12, thanks.
> 
> -- 
> Thanks,
> Sasha

Phew, good to hear, thank you!

There might be for some time more than usual merge conflicts in TPM
driver given making tpm_buf memory layout flat for the sake of being
able to use __free for of its allocations, and thus completely 
prevent any possible memory leaks from transient buffers.

Right, and also merge conflicts could happen also in trusted keys.

They are cheap to fix and I'll monitor this situation proactively,
and try to react fast.

Br, Jarkko