| Message ID | 20230803015015.915-3-mario.limonciello@amd.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | More changes related to TPM RNG handling | expand |
On Wed, Aug 02, 2023 at 08:50:14PM -0500, Mario Limonciello wrote: > The kernel supports random.cpu=off and random.bootloader=off. > As TPM RNG is also registered as a hwrng, add the ability to > prevent registering the TPM RNG. Please do *not* do this. I agree with Jarkko that this doesn't belong. Firstly, you're proposing a flag for the tpm driver, so the `random.` namespace is inappropriate. Do not use the `random.` namespace if you're not dealing with random.c specifically. Rather, this is very much a `tpm.register_hwrng=1/0` flag, which describes better what this is about. Secondly, I think you're making a mountain out of a molehill. You first wanted to also disable Intel devices too, even though they aren't affected by this bug. Now you're proposing a way for users to disable everything. But so far there's no evidence that this matter goes any further than AMD's fTPM. So let's calm a bit and not make too big deal of this. If we suddenly get lots of reports that there's broken behavior across the board, then maybe we should consider something like this. But insofar as this is just an AMD derp, let's keep it simple and not over complicate everything with more knobs. Fewer knobs, please! Finally, with regards to AMD, my hope is that eventually the fTPM becomes useful as a hwrng, and then we can relax the disabling to re-enable it for whatever new revision might come to exist in the future. Thanks, Jason
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index a1457995fd41c..9ff602c09f55c 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4672,6 +4672,11 @@ passed by the bootloader (if available) to initialize the kernel's RNG. + random.trust_tpm=off + [KNL] Disable trusting the use of the TPM's + random number generator (if available) to + initialize the kernel's RNG. + randomize_kstack_offset= [KNL] Enable or disable kernel stack offset randomization, which provides roughly 5 bits of diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c index 8f61b784810d6..8fb42232bd7a5 100644 --- a/drivers/char/tpm/tpm-chip.c +++ b/drivers/char/tpm/tpm-chip.c @@ -32,6 +32,13 @@ struct class *tpm_class; struct class *tpmrm_class; dev_t tpm_devt; +static bool trust_tpm __initdata = true; +static int __init parse_trust_tpm(char *arg) +{ + return kstrtobool(arg, &trust_tpm); +} +early_param("random.trust_tpm", parse_trust_tpm); + static int tpm_request_locality(struct tpm_chip *chip) { int rc; @@ -523,6 +530,9 @@ static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait) static int tpm_add_hwrng(struct tpm_chip *chip) { + if (!trust_tpm) + chip->flags |= TPM_CHIP_FLAG_HWRNG_DISABLED; + if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM) || tpm_is_firmware_upgrade(chip) || chip->flags & TPM_CHIP_FLAG_HWRNG_DISABLED) return 0;
The kernel supports random.cpu=off and random.bootloader=off. As TPM RNG is also registered as a hwrng, add the ability to prevent registering the TPM RNG. Suggested-by: Mateusz Schyboll <dragonn@op.pl> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com> --- Documentation/admin-guide/kernel-parameters.txt | 5 +++++ drivers/char/tpm/tpm-chip.c | 10 ++++++++++ 2 files changed, 15 insertions(+)