Re: [PATCH 10/18] crypto: dh - introduce support for ephemeral key generation to dh-generic

[Nicolai Stange <nstange@suse.de>]

Stephan Müller <smueller@chronox.de> writes:

> Am Mittwoch, 1. Dezember 2021, 01:48:50 CET schrieb Nicolai Stange:
>
> Hi Nicolai,
>
>> The support for NVME in-band authentication currently in the works ([1])
>> needs to generate ephemeral DH keys. Make dh-generic's ->set_secret()
>> to generate an ephemeral key via the recently added crypto_dh_gen_privkey()
>> in case the input ->key_size is zero. Note that this behaviour is in
>> analogy to ecdh's ->set_secret().
>> 
>> [1] https://lkml.kernel.org/r/20211122074727.25988-4-hare@suse.de
>> 
>> Signed-off-by: Nicolai Stange <nstange@suse.de>
>> ---
>>  crypto/dh.c | 24 ++++++++++++++++++++----
>>  1 file changed, 20 insertions(+), 4 deletions(-)
>> 
>> diff --git a/crypto/dh.c b/crypto/dh.c
>> index 131b80064cb1..2e49b114e038 100644
>> --- a/crypto/dh.c
>> +++ b/crypto/dh.c
>> @@ -71,25 +71,41 @@ static int dh_set_secret(struct crypto_kpp *tfm, const
>> void *buf, {
>>  	struct dh_ctx *ctx = dh_get_ctx(tfm);
>>  	struct dh params;
>> +	char key[CRYPTO_DH_MAX_PRIVKEY_SIZE];
>> +	int err;
>> 
>>  	/* Free the old MPI key if any */
>>  	dh_clear_ctx(ctx);
>> 
>> -	if (crypto_dh_decode_key(buf, len, &params) < 0)
>> +	err = crypto_dh_decode_key(buf, len, &params);
>> +	if (err)
>>  		goto err_clear_ctx;
>> 
>> -	if (dh_set_params(ctx, &params) < 0)
>> +	if (!params.key_size) {
>
> As this params data may come from user space, shouldn't we use the same logic 
> as in ecdh's set_key function:
>
> if (!params.key || !params.key_size)

crypto_dh_decode_key() always leaves params.key set even for
!params.key_size, so checking for !params.key wouldn't buy anything
here. FWIW, it seems like it's actually the same for
crypto_ecdh_decode_key().

I'd personally prefer to not add the !params.key check, because it would
suggest that there are code paths which can lead to the condition
params.key_size && !params.key. I would find this confusing when reading
the code, but OTOH I don't have strong objections, so if you insist on
adding the !params.key check, I'd be Ok with it.

Thanks,

Nicolai

>
> ?
>
>
>> +		err = crypto_dh_gen_privkey(params.group_id, key,
>> +					    &params.key_size);
>> +		if (err)
>> +			goto err_clear_ctx;
>> +		params.key = key;
>> +	}
>> +
>> +	err = dh_set_params(ctx, &params);
>> +	if (err)
>>  		goto err_clear_ctx;
>> 
>>  	ctx->xa = mpi_read_raw_data(params.key, params.key_size);
>> -	if (!ctx->xa)
>> +	if (!ctx->xa) {
>> +		err = -EINVAL;
>>  		goto err_clear_ctx;
>> +	}
>> +
>> +	memzero_explicit(key, sizeof(key));
>> 
>>  	return 0;
>> 
>>  err_clear_ctx:
>>  	dh_clear_ctx(ctx);
>> -	return -EINVAL;
>> +	return err;
>>  }
>> 
>>  /*
>
>
> Ciao
> Stephan
>
>

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