linux-kernel - Re: [PATCH v2 4/6] KEYS: trusted: Move tpm2_key

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Date: Tue, 21 May 2024 14:18:45 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, Herbert Xu
	 <herbert@...dor.apana.org.au>
Cc: linux-integrity@...r.kernel.org, keyrings@...r.kernel.org, 
 Andreas.Fuchs@...ineon.com, James Prestwood <prestwoj@...il.com>, David
 Woodhouse <dwmw2@...radead.org>, Eric Biggers <ebiggers@...nel.org>, "David
 S. Miller" <davem@...emloft.net>, "open list:CRYPTO API"
 <linux-crypto@...r.kernel.org>,  open list <linux-kernel@...r.kernel.org>,
 Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>, Mimi Zohar
 <zohar@...ux.ibm.com>, David Howells <dhowells@...hat.com>, Paul Moore
 <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn"
 <serge@...lyn.com>, "open list:SECURITY SUBSYSTEM"
 <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v2 4/6] KEYS: trusted: Move tpm2_key_decode() to the TPM
 driver

On Tue, 2024-05-21 at 06:16 +0300, Jarkko Sakkinen wrote:
[...]
> diff --git a/include/crypto/tpm2_key.h b/include/crypto/tpm2_key.h
> new file mode 100644
> index 000000000000..acf41b2e0c92
> --- /dev/null
> +++ b/include/crypto/tpm2_key.h
> @@ -0,0 +1,33 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef __LINUX_TPM2_KEY_H__
> +#define __LINUX_TPM2_KEY_H__
> +
> +#include <linux/slab.h>
> +
> +/*
> + * TPM2 ASN.1 key
> + */
> +struct tpm2_key {
> +       u32 parent;
> +       const u8 *blob;
> +       u32 blob_len;
> +       const u8 *pub;
> +       u32 pub_len;
> +       const u8 *priv;
> +       u32 priv_len;
> +};
> +
> +int tpm2_key_decode(const u8 *src, u32 src_len, struct tpm2_key
> *key,
> +                   u32 max_key_len);

I don't think this is a good idea.  Trusted keys already have a pre-
defined max payload size (MAX_BLOB_SIZE in include/keys/trusted-type.h)
and I've already had to increase this several times because once you
get policy attached to a key, it can get pretty big (over a page). 
Exactly the same thing will happen to asymmetric keys as well, so it
does make sense that they share the same maximum (probably in a more
generic header, though).

Since the code already right sizes the allocation and all we check with
this is whether it's over a pre-defined maximum, it's way easier if
that maximum is defined in a header rather than passed in in several
places making increasing the maximum really hard because you have to
chase all the threading.

James