From nobody Thu Dec 18 19:59:24 2025 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E1FEA146A66; Mon, 15 Dec 2025 23:50:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765842604; cv=none; b=ZCLYsr3NidTIqbo8AHkmpRNnUPN5D02yGbvjc1AjrD3r2ePBHAFCW7hC1VITFOYYFqpQrNO/MCJ85GiCGbiJUo93EZf1BbGVdjyxVmT8w8ogmxQkBd0A5kA1zEGNSjwlRZ7B8WTVjFy9fkcc7exEv1VJkSpFasvqCssMg33nYJk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765842604; c=relaxed/simple; bh=5x88HEIkTgYf1xD7GovmrI5/UkL+Hu1QJ4lkwsKBz+I=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=SdStvJ+EqYwhSMVjlH9ig+ayjKzibvbqTWjC8WqaPwCucWWBtANcG9UZn5NNVHbe7BMzir3zZW+Lehry8KpwnHrqBEa8DFAL5zLAQCFVuyUdwJXuEMC5SQpMXMCC+G/SKf6ui61b7ajRKJbO6GNF+JOXfoQqk0SupfAoU5Cm6wo= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Crnh2DGm; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Crnh2DGm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E788CC4CEF5; Mon, 15 Dec 2025 23:50:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1765842602; bh=5x88HEIkTgYf1xD7GovmrI5/UkL+Hu1QJ4lkwsKBz+I=; h=From:To:Cc:Subject:Date:From; b=Crnh2DGmFv5/LJoE3V1lqZ7itegKM1oPvG+UaRL9ZkZCGuHERR75/GzfotBNia8T0 DSU37glOuBAFTrNHnX3aspMh+s3ggVzHxNoULAskPOUo1R2QyJan0OIs5CVGW7MgQK z3j6rOXXKOYyOUx6RUX30T0tnTRZogXNzwYTNOBjHfO8gI6UI2KT4mafpzS3I8EvVt zz/c4q93B03Gf8a4CuvYxxhrsGsd58UG6BIZDtTbgna+7CnnE/z8teVFhlRVVwHpPn S0jaeCZJmAlaMo9DsU8ewkM9LOGr0ItyXNIBER1TzXI431mU7s4YiXsBgkYgEAeN4e xyF6OmQqH21UA== From: Jarkko Sakkinen To: linux-integrity@vger.kernel.org Cc: Jarkko Sakkinen , Eric Biggers , David Howells , Paul Moore , James Morris , "Serge E. Hallyn" , James Bottomley , Mimi Zohar , keyrings@vger.kernel.org (open list:KEYS/KEYRINGS), linux-security-module@vger.kernel.org (open list:SECURITY SUBSYSTEM), linux-kernel@vger.kernel.org (open list) Subject: [PATCH v3] KEYS: trusted: Use get_random-fallback for TPM Date: Tue, 16 Dec 2025 01:49:54 +0200 Message-Id: <20251215234954.607848-1-jarkko@kernel.org> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 1. tpm2_get_random() is costly when TCG_TPM2_HMAC is enabled and thus its use should be pooled rather than directly used. This both reduces latency and improves its predictability. 2. Linux is better off overall if every subsystem uses the same source for generating the random numbers required. Thus, unset '.get_random', which causes fallback to kernel_get_random(). One might argue that TPM RNG should be used for the generated trusted keys, so that they have matching entropy with the TPM internally generated objects. This argument does have some weight into it but as far cryptography goes, FIPS certification sets the exact bar, not which exact FIPS certified RNG will be used. Thus, the rational choice is obviously to pick the lowest latency path, which is kernel RNG. Finally, there is an actual defence in depth benefit when using kernel RNG as it helps to mitigate TPM firmware bugs concerning RNG implementation, given the obfuscation by the other entropy sources. Reviewed-by: Eric Biggers Signed-off-by: Jarkko Sakkinen --- v3: - Fixed typos in the commit message. - Moved the documentation comment to the correct location. v2: - Added Eric's reviewed-by tag. - Addressed concerns from James by writing more details to the commit message and documenting random number generation to the source code. --- security/keys/trusted-keys/trusted_tpm1.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/security/keys/trusted-keys/trusted_tpm1.c b/security/keys/trus= ted-keys/trusted_tpm1.c index 636acb66a4f6..7ce7e31bcdfb 100644 --- a/security/keys/trusted-keys/trusted_tpm1.c +++ b/security/keys/trusted-keys/trusted_tpm1.c @@ -6,6 +6,16 @@ * See Documentation/security/keys/trusted-encrypted.rst */ =20 +/** + * DOC: Random Number Generation + * + * tpm_get_random() was previously used here as the RNG in order to have e= qual + * entropy with the objects fully inside the TPM. However, as far as goes, + * kernel RNG is equally fine, as long as long as it is FIPS certified. Al= so, + * using kernel RNG has the benefit of mitigating bugs in the TPM firmware + * associated with the RNG. + */ + #include #include #include @@ -936,11 +946,6 @@ static int trusted_tpm_unseal(struct trusted_key_paylo= ad *p, char *datablob) return ret; } =20 -static int trusted_tpm_get_random(unsigned char *key, size_t key_len) -{ - return tpm_get_random(chip, key, key_len); -} - static int __init init_digests(void) { int i; @@ -992,6 +997,5 @@ struct trusted_key_ops trusted_key_tpm_ops =3D { .init =3D trusted_tpm_init, .seal =3D trusted_tpm_seal, .unseal =3D trusted_tpm_unseal, - .get_random =3D trusted_tpm_get_random, .exit =3D trusted_tpm_exit, }; --=20 2.39.5