Re: [PATCH v2 4/6] KEYS: trusted: Move tpm2_key_decode() to the TPM driver

[James Bottomley]

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