Re: [PATCH v8 18/22] tpm: add session encryption protection to tpm2_get_random()

Nícolas F. R. A. Prado
2024-05-17T16:22:52Z

On Fri, May 17, 2024 at 07:25:40AM -0700, James Bottomley wrote:
> On Fri, 2024-05-17 at 15:43 +0200, Ard Biesheuvel wrote:
> > On Fri, 17 May 2024 at 15:35, James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> [...]
> > > Thanks for the analysis.  If I look at how CRYPTO_ECC does it, that
> > > selects CRYPTO_RNG_DEFAULT which pulls in CRYPTO_DRBG, so the fix
> > > would be the attached.  Does that look right to you Ard?
> >
> > No it doesn't - it's CRYPTO_RNG_DEFAULT not CRYTPO_RNG_DEFAULT :-)
> >
> > With that fixed,
> >
> > Acked-by: Ard Biesheuvel <ardb@kernel.org>
>
> Erm, oops, sorry about that; so attached is the update.
>
> James
>
> ---8>8>8><8<8<8---
>
> From 2ac337a33e6416ef806e2c692b9239d193e8468f Mon Sep 17 00:00:00 2001
> From: James Bottomley <James.Bottomley@HansenPartnership.com>
> Date: Fri, 17 May 2024 06:29:31 -0700
> Subject: [PATCH] tpm: Fix sessions cryptography requirement for Random Numbers
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> The ECDH code in tpm2-sessions.c requires an initial random number
> generator to generate the key pair. If the configuration doesn't have
> CONFIG_RNG_DEFAULT, it will try to pull this in as a module (which is
> impossible for the early kernel boot where the TPM starts). Fix this
> by selecting the required RNG.
>
> Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> Fixes: 1b6d7f9eb150 ("tpm: add session encryption protection to tpm2_get_random()")
> Acked-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> ---
> drivers/char/tpm/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> index 4f83ee7021d0..ecdd3db4be2b 100644
> --- a/drivers/char/tpm/Kconfig
> +++ b/drivers/char/tpm/Kconfig
> @@ -31,6 +31,7 @@ config TCG_TPM2_HMAC
> bool "Use HMAC and encrypted transactions on the TPM bus"
> default y
> select CRYPTO_ECDH
> + select CRYPTO_RNG_DEFAULT
> select CRYPTO_LIB_AESCFB
> select CRYPTO_LIB_SHA256
> help
> --
> 2.35.3
>
>

Hi James,

thanks for the patch. But I actually already had that config enabled builtin. I
also had ECDH and DRBG which have been suggested previously:

CONFIG_CRYPTO_RNG_DEFAULT=y

CONFIG_CRYPTO_DRBG_MENU=y
CONFIG_CRYPTO_DRBG_HMAC=y
# CONFIG_CRYPTO_DRBG_HASH is not set
# CONFIG_CRYPTO_DRBG_CTR is not set
CONFIG_CRYPTO_DRBG=y

CONFIG_CRYPTO_ECDH=y

I've pasted my full config here: http://0x0.st/XPN_.txt

Adding a debug print I see that the module that the code tries to load is
"crypto-hmac(sha512)". I would have expected to see

MODULE_ALIAS_CRYPTO("hmac(sha512)");

in crypto/drbg.c, but I don't see it anywhere in the tree. Maybe it is missing?

Thanks,
Nícolas