Re: SLB9670 TPM module crash

[Jarkko Sakkinen]

On Sat May 18, 2024 at 2:21 PM EEST, Parthiban wrote:
> Dear James Bottomley,
>
> The following crash is observed in the current mainline kernel and I have tried the
> git bisect to narrow it down. Bisect points to the below commit, which got merged as
> part of [1]. I tried reverting the below commit and the TPM loads fine.
>
> commit 1b6d7f9eb150305dcb0da4f7101a8d30dcdf0497
> Author: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Date:   Mon Apr 29 16:28:07 2024 -0400
>
>     tpm: add session encryption protection to tpm2_get_random()
>     
>     If some entity is snooping the TPM bus, they can see the random
>     numbers we're extracting from the TPM and do prediction attacks
>     against their consumers.  Foil this attack by using response
>     encryption to prevent the attacker from seeing the random sequence.
>     
>     Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
>     Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
>     Tested-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
>     Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
>
>  drivers/char/tpm/tpm2-cmd.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
>
> [   11.551988] tpm_tis_spi spi0.1: 2.0 TPM (device-id 0x1B, rev-id 22)
> [   11.563036] spi_master spi0: will run message pump with realtime priority


Explanation and workaround: https://lore.kernel.org/linux-integrity/D1C1KL7Q27P9.39BH0Z4EMBBUG@xxxxxxxxxx/

James, this must be fixed by:

diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
index 7c0486e3199c..2d9e2c860ad9 100644
--- a/drivers/char/tpm/Kconfig
+++ b/drivers/char/tpm/Kconfig
@@ -34,7 +34,7 @@ if TCG_TPM

 config TCG_TPM2_HMAC
        bool "Use HMAC and encrypted transactions on the TPM bus"
-       default y
+       default n
        select CRYPTO_ECDH
        select CRYPTO_LIB_AESCFB
        select CRYPTO_LIB_SHA256

Distributors know how to enable this but given the high volumes of small
devices still with TPM, this trend needs to be cutted.

BR, Jarkko


BR, Jarkko