Re: [PATCH v6 3/6] crypto: AF_ALG -- add asymmetric cipher interface

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Wed, 8 Jun 2016, Stephan Mueller wrote:

> Am Dienstag, 7. Juni 2016, 17:28:07 schrieb Mat Martineau:
>
> Hi Mat,
>
>>> +	used = ctx->used;
>>> +
>>> +	/* convert iovecs of output buffers into scatterlists */
>>> +	while (iov_iter_count(&msg->msg_iter)) {
>>> +		/* make one iovec available as scatterlist */
>>> +		err = af_alg_make_sg(&ctx->rsgl[cnt], &msg->msg_iter,
>>> +				     iov_iter_count(&msg->msg_iter));
>>> +		if (err < 0)
>>> +			goto unlock;
>>> +		usedpages += err;
>>> +		/* chain the new scatterlist with previous one */
>>> +		if (cnt)
>>> +			af_alg_link_sg(&ctx->rsgl[cnt - 1], &ctx->rsgl[cnt]);
>>> +
>>> +		iov_iter_advance(&msg->msg_iter, err);
>>> +		cnt++;
>>> +	}
>>> +
>>> +	/* ensure output buffer is sufficiently large */
>>> +	if (usedpages < akcipher_calcsize(ctx)) {
>>> +		err = -EMSGSIZE;
>>> +		goto unlock;
>>> +	}
>>
>> Why is the size of the output buffer enforced here instead of depending on
>> the algorithm implementation?
>
> akcipher_calcsize calls crypto_akcipher_maxsize to get the maximum size the
> algorithm generates as output during its operation.
>
> The code ensures that the caller provided at least that amount of memory for
> the kernel to store its data in. This check therefore is present to ensure the
> kernel does not overstep memory boundaries in user space.

Yes, it's understood that the userspace buffer length must not be 
exceeded. But dst_len is part of the akcipher_request struct, so why does 
it need to be checked *here* when it is also checked later?

> What is your concern?

Userspace must allocate larger buffers than it knows are necessary for 
expected results.

It looks like the software rsa implementation handles shorter output 
buffers ok (mpi_write_to_sgl will return EOVERFLOW if the the buffer is 
too small), however I see at least one hardware rsa driver that requires 
the output buffer to be the maximum size. But this inconsistency might be 
best addressed within the software cipher or drivers rather than in 
recvmsg.

--
Mat Martineau
Intel OTC