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tpm: fix off-by-four bounds check in tpm2_get_random()

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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: 1b6d7f9eb1 ("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>
This commit is contained in:
Jiangshan Yi 2026-09-03 11:58:37 +08:00 committed by Jarkko Sakkinen
parent 1e2f9a611b
commit 9b522400bf
1 changed files with 1 additions and 1 deletions
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2
drivers/char/tpm/tpm2-cmd.c
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@ -307,7 +307,7 @@ 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) {
tpm2_end_auth_session(chip);