linux-kernel - Re: [PATCH 2/2] crypto: asymmetric_keys - simplify asymmetric_key_hex_to_key

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Message-ID: <aOuavtSpoNLWHoMS@wunner.de>
Date: Sun, 12 Oct 2025 14:10:38 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Thorsten Blum <thorsten.blum@...ux.dev>
Cc: David Howells <dhowells@...hat.com>,
	Ignat Korchagin <ignat@...udflare.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, keyrings@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] crypto: asymmetric_keys - simplify
 asymmetric_key_hex_to_key_id

On Tue, Oct 07, 2025 at 08:52:21PM +0200, Thorsten Blum wrote:
> Use struct_size() to calculate the number of bytes to allocate for the
> asymmetric key id.

Why?  To what end?  To guard against an overflow?

I've gone through the callers of asymmetric_key_hex_to_key_id() and
it seems they all limit the length of the keyid.  Guarding against
an overflow in asymmetric_key_hex_to_key_id() could thus only be
justified as a defense-in-depth measure, but I doubt that's worth it.

Callers I've found:

- keyctl_keyring_search() [security/keys/keyctl.c]
    keyring_search()
      type->match_preparse() == asymmetric_key_match_preparse()
        asymmetric_key_hex_to_key_id()

  Here the size of the key id is constrained to 4096 bytes
  (KEY_MAX_DESC_SIZE) by keyctl_keyring_search().

- request_key() [security/keys/keyctl.c]
    request_key_and_link()
        type->match_preparse() == asymmetric_key_match_preparse()
          asymmetric_key_hex_to_key_id()

  Same here.

- asymmetric_verify() [security/integrity/digsig_asymmetric.c]
    request_asymmetric_key()
      find_asymmetric_key()
        keyring_search()
          type->match_preparse() == asymmetric_key_match_preparse()
            asymmetric_key_hex_to_key_id()

  Here the size of the key id is a 32-bit integer.

- pkcs7_validate_trust_one() [crypto/asymmetric_keys/pkcs7_trust.c]
    find_asymmetric_key()
      keyring_search()
        type->match_preparse() == asymmetric_key_match_preparse()
          asymmetric_key_hex_to_key_id()

  Here the key id in hexadecimal is constructed from its binary form,
  asymmetric_key_hex_to_key_id() then converts that back.  Naturally
  the back conversion shouldn't overflow.

- restrict_link_by_signature() [crypto/asymmetric_keys/restrict.c]
    find_asymmetric_key()
      keyring_search()
        type->match_preparse() == asymmetric_key_match_preparse()
          asymmetric_key_hex_to_key_id()

  Same here.

> +++ b/crypto/asymmetric_keys/asymmetric_type.c
> @@ -236,12 +236,11 @@ struct asymmetric_key_id *asymmetric_key_hex_to_key_id(const char *id)
>  	if (asciihexlen & 1)
>  		return ERR_PTR(-EINVAL);
>  
> -	match_id = kmalloc(sizeof(struct asymmetric_key_id) + asciihexlen / 2,
> -			   GFP_KERNEL);
> +	hexlen = asciihexlen / 2;
> +	match_id = kmalloc(struct_size(match_id, data, hexlen), GFP_KERNEL);
>  	if (!match_id)
>  		return ERR_PTR(-ENOMEM);

This doesn't look more readable to be honest.

> -	ret = __asymmetric_key_hex_to_key_id(id, match_id, asciihexlen / 2);
> -	if (ret < 0) {
> +	if (__asymmetric_key_hex_to_key_id(id, match_id, hexlen) < 0) {
>  		kfree(match_id);
>  		return ERR_PTR(-EINVAL);
>  	}

If anything, return ret instead of removing the ret variable.
The only negative return value of __asymmetric_key_hex_to_key_id()
is -EINVAL, hence that's returned directly here.

Thanks,

Lukas