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

[Lukas Wunner <lukas@wunner.de>]

On Mon, Oct 13, 2025 at 10:23:01AM +0200, Thorsten Blum wrote:
> On 13. Oct 2025, at 08:24, Lukas Wunner wrote:
> > On Sun, Oct 12, 2025 at 10:38:40PM +0200, Thorsten Blum wrote:
> >> +++ 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).
> 
> Could you explain which part adds "(at least) 2 bytes to len"?

The struct_size() macro performs another size_add() to add the
size of struct asymmetric_key_id (which is at least 2 bytes) to len:

#define struct_size(p, member, count)					\
	__builtin_choose_expr(__is_constexpr(count),			\
		sizeof(*(p)) + flex_array_size(p, member, count),	\
		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
                ^^^^^^^^

So there's an addition of three numbers, yet you're only checking that
the addition of two of them doesn't overflow.

Thanks,

Lukas