From: Jan Kiszka <[email protected]> As seen with optee_ftpm, which uses ms-tpm-20-ref [1], a TPM may write the current time epoch to its NV storage every 4 seconds if there are commands sent to it. The 60 seconds periodic update of the entropy pool that the hwrng kthread does triggers this, causing about 4 writes per requests. Makes 2 millions per year for a 24/7 device, and that is a lot for its backing NV storage.
It is therefore better to make the user intentionally enable this, providing a chance to read the warning. [1] https://github.com/Microsoft/ms-tpm-20-ref Signed-off-by: Jan Kiszka <[email protected]> --- drivers/char/tpm/Kconfig | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig index 8a8f692b6088..d64c929cacbe 100644 --- a/drivers/char/tpm/Kconfig +++ b/drivers/char/tpm/Kconfig @@ -45,13 +45,17 @@ config TCG_TPM2_HMAC config HW_RANDOM_TPM bool "TPM HW Random Number Generator support" depends on TCG_TPM && HW_RANDOM && !(TCG_TPM=y && HW_RANDOM=m) - default y help This setting exposes the TPM's Random Number Generator as a hwrng device. This allows the kernel to collect randomness from the TPM at boot, and provides the TPM randomines in /dev/hwrng. - If unsure, say Y. + WARNING: Specifically firmware-based TPMs, possibly also hardware + variants, can wear-out from the frequent requests issued by the + Hardware Random Number Generator Core when filling the kernel's + entropy pool. These requests are sent once every minute by default, + and the TPM may write the current time to its NV storage for each of + them. config TCG_TIS_CORE tristate -- 2.51.0
