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

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Date:	Thu, 16 Jun 2016 17:38:14 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Andrew Zaborowski <balrogg@...glemail.com>
Cc:	Mat Martineau <mathew.j.martineau@...ux.intel.com>,
	Tadeusz Struk <tadeusz.struk@...el.com>,
	David Howells <dhowells@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	linux-api@...r.kernel.org, Marcel Holtmann <marcel@...tmann.org>,
	linux-kernel@...r.kernel.org, keyrings@...r.kernel.org,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>, davem@...emloft.net
Subject: Re: [PATCH v6 3/6] crypto: AF_ALG -- add asymmetric cipher interface

Am Donnerstag, 16. Juni 2016, 16:59:01 schrieb Andrew Zaborowski:

Hi Andrew,

> Hi Stephan,
> 
> On 16 June 2016 at 10:05, Stephan Mueller <smueller@...onox.de> wrote:
> > Am Dienstag, 14. Juni 2016, 09:42:34 schrieb Andrew Zaborowski:
> > 
> > Hi Andrew,
> > 
> >> > I think we have agreed on dropping the length enforcement at the
> >> > interface
> >> > level.
> >> 
> >> Separately from this there's a problem with the user being unable to
> >> know if the algorithm is going to fail because of destination buffer
> >> size != key size (including kernel users).  For RSA, the qat
> >> implementation will fail while the software implementation won't.  For
> >> pkcs1pad(...) there's currently just one implementation but the user
> >> can't assume that.
> > 
> > If I understand your issue correctly, my initial code requiring the caller
> > to provide sufficient memory would have covered the issue, right?
> 
> This isn't an issue with AF_ALG, I should have changed the subject
> line perhaps.  In this case it's an inconsistency between some
> implementations and the documentation (header comment).  It affects
> users accessing the cipher through AF_ALG but also directly.

As I want to send a new version of the algif_akcipher shortly now (hoping for 
an inclusion into 4.8), is there anything you see that I should prepare for 
regarding this issue? I.e. do you forsee a potential fix that would change the 
API or ABI of algif_akcipher?
> 
> > If so, we seem
> > to have implementations which can handle shorter buffer sizes and some
> > which do not. Should a caller really try to figure the right buffer size
> > out? Why not requiring a mandatory buffer size and be done with it? I.e.
> > what is the gain to allow shorter buffer sizes (as pointed out by Mat)?
> 
> It's that client code doesn't need an intermediate layer with an
> additional buffer and a memcpy to provide a sensible API.  If the code
> wants to decrypt a 32-byte Digest Info structure with a given key or a
> reference to a key it makes no sense, logically or in terms of
> performance, for it to provide a key-sized buffer.
> 
> In the case of the userspace interface I think it's also rare for a
> recv() or read() on Linux to require a buffer larger than it's going
> to use, correct me if i'm wrong.  (I.e. fail if given a 32-byte
> buffer, return 32 bytes of data anyway)  Turning your questino around
> is there a gain from requiring larger buffers?

That is a good one :-)

I have that check removed.

Ciao
Stephan