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

James Bottomley
2024-05-17T13:35:25Z

On Fri, 2024-05-17 at 09:20 +0200, Ard Biesheuvel wrote:
> On Fri, 17 May 2024 at 03:59, James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> >
> > On Thu, 2024-05-16 at 20:25 -0400, Nícolas 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.

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? And does it work
Nicolas?

James

---8>8>8><8<8<8---

From 8c60ffd959eaa65627aca6596c35bb9cbfd9bee6 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()")
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..12065eddb922 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 CRYTPO_RNG_DEFAULT
select CRYPTO_LIB_AESCFB
select CRYPTO_LIB_SHA256
help
--
2.35.3