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From: Ard Biesheuvel <ardb@kernel.org>
Date: Fri, 17 May 2024 09:20:49 +0200
Message-ID: <CAMj1kXHi4r8KY9GvX573kwqvLpMfX-J=K2hWiGAKkf5bnicwYQ@mail.gmail.com>
Subject: Re: [PATCH v8 18/22] tpm: add session encryption protection to tpm2_get_random()
To: James Bottomley <James.Bottomley@hansenpartnership.com>, 
	Linux Crypto Mailing List <linux-crypto@vger.kernel.org>, Herbert Xu <herbert@gondor.apana.org.au>
Cc: =?UTF-8?B?TsOtY29sYXMgRi4gUi4gQS4gUHJhZG8=?= <nfraprado@collabora.com>, 
	linux-integrity@vger.kernel.org, Jarkko Sakkinen <jarkko@kernel.org>, 
	keyrings@vger.kernel.org, regressions@lists.linux.dev, kernel@collabora.com
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On Fri, 17 May 2024 at 03:59, James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Thu, 2024-05-16 at 20:25 -0400, N=C3=ADcolas F. R. A. Prado wrote:
...
> > KernelCI has identified a new warning and I tracked it down to this
> > commit. It
> > was observed on the following platforms:
> > * mt8183-kukui-jacuzzi-juniper-sku16
> > * sc7180-trogdor-kingoftown
> > (but probably affects all platforms that have a tpm driver with async
> > probe)
> >
> > [    2.175146] Call trace:
> > [    2.177587]  __request_module+0x188/0x1f4
> > [    2.181596]  crypto_alg_mod_lookup+0x178/0x21c
> > [    2.186042]  crypto_alloc_tfm_node+0x58/0x114
> > [    2.190396]  crypto_alloc_shash+0x24/0x30
> > [    2.194404]  drbg_init_hash_kernel+0x28/0xdc
> > [    2.198673]  drbg_kcapi_seed+0x21c/0x420
> > [    2.202593]  crypto_rng_reset+0x84/0xb4
> > [    2.206425]  crypto_get_default_rng+0xa4/0xd8
> > [    2.210779]  ecc_gen_privkey+0x58/0xd0
> > [    2.214526]  ecdh_set_secret+0x90/0x198
> > [    2.218360]  tpm_buf_append_salt+0x164/0x2dc
>
> This looks like a misconfiguration.  The kernel is trying to load the
> ecdh module, but it should have been selected as built in by this in
> drivers/char/tpm/Kconfig:
>
> config TCG_TPM2_HMAC
>         bool "Use HMAC and encrypted transactions on the TPM bus"
>         default y
>         select CRYPTO_ECDH
>         select CRYPTO_LIB_AESCFB
>         select CRYPTO_LIB_SHA256
>

The module request is not for ECDH itself but for the DRBG it attempts
to use to generate the secret.

Given that CRYPTO_ECDH does not strictly require a DRBG in principle,
but does in this particular case, I think it makes sense to select
CRYPTO_DRBG here (or depend on it being builtin), rather than updating
the Kconfig rules for CRYPTO_ECDH itself.