Re: [PATCH v2 1/2] crypto: asymmetric_keys - prevent overflow in asymmetric_key_generate_id

[Lukas Wunner <lukas@wunner.de>]

On Sun, Oct 12, 2025 at 10:38:40PM +0200, Thorsten Blum wrote:
> Use check_add_overflow() to guard against a potential integer overflow
> when adding the binary blob lengths in asymmetric_key_generate_id() and
> return -EOVERFLOW accordingly. This prevents a possible buffer overflow
> when copying data from potentially malicious X.509 fields that can be
> arbitrarily large, such as ASN.1 INTEGER serial numbers, issuer names,
> etc.
> 
> Also use struct_size() to calculate the number of bytes to allocate for
> the new asymmetric key id.
> 
> Cc: stable@vger.kernel.org
> Fixes: 7901c1a8effb ("KEYS: Implement binary asymmetric key ID handling")

No stable designation please, this doesn't pass the "obviously correct"
test, see below.

> +++ b/crypto/asymmetric_keys/asymmetric_type.c
> @@ -141,12 +142,14 @@ struct asymmetric_key_id *asymmetric_key_generate_id(const void *val_1,
>  						     size_t len_2)
>  {
>  	struct asymmetric_key_id *kid;
> +	size_t len;
>  
> -	kid = kmalloc(sizeof(struct asymmetric_key_id) + len_1 + len_2,
> -		      GFP_KERNEL);
> +	if (check_add_overflow(len_1, len_2, &len))
> +		return ERR_PTR(-EOVERFLOW);
> +	kid = kmalloc(struct_size(kid, data, len), GFP_KERNEL);

This will add (at least) 2 bytes to len (namely the size of struct
asymmetric_key_id)) and may cause an overflow (even if len_1 + len_2
did not overflow).

struct_size() truncates to SIZE_MAX and then right below...

>  	if (!kid)
>  		return ERR_PTR(-ENOMEM);
> -	kid->len = len_1 + len_2;
> +	kid->len = len;
>  	memcpy(kid->data, val_1, len_1);
>  	memcpy(kid->data + len_1, val_2, len_2);

... this memcpy() operation will perform an out-of-bound access
beyond SIZE_MAX.

Thanks,

Lukas