Re: [RFC PATCH 2/2] Crypto kernel tls socket

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

Hello,

Stephan Mueller <smueller@chronox.de> writes:

> Am Dienstag, 24. November 2015, 18:34:55 schrieb Herbert Xu:
>
> Hi Herbert,
>
>>On Mon, Nov 23, 2015 at 09:43:02AM -0800, Dave Watson wrote:
>>> Userspace crypto interface for TLS.  Currently supports gcm(aes) 128bit
>>> only, however the interface is the same as the rest of the SOCK_ALG
>>> interface, so it should be possible to add more without any user interface
>>> changes.
>>
>>SOCK_ALG exists to export crypto algorithms to user-space.  So if
>>we decided to support TLS as an algorithm then I guess this makes
>>sense.
>>
>>However, I must say that it wouldn't have been my first pick.  I'd
>>imagine a TLS socket to look more like a TCP socket, or perhaps a
>>KCM socket as proposed by Tom.
>
> If I may ask: what is the benefit of having TLS in kernel space? I do not see 
> any reason why higher-level protocols should be in the kernel as they do not 
> relate to accessing hardware.

There are some crypto acclerators out there so that putting tls into the
kernel would give a net benefit, because otherwise user space has to
copy data into the kernel for device access and back to user space until
it can finally be send out on the wire.

Since processors provide aesni and other crypto extensions as part of
their instruction set architecture, this, of course, does not make sense
any more.

> The only reason I could fathom is to keep the negotiated keys in a secure 
> realm. But that could be done without having parts or the whole TLS protocol 
> stack in the kernel. If the key management should stay in the kernel as a 
> protection domain, I would rather think that the kernel should offer a plug-
> and-play of raw ciphers where user space is responsible to form a protocol. 
> This way, we do not limit such key management to TLS, but allow any kind of 
> protocol to use it.
>
> E.g. the kernel performs the key transport with RSA or key agreement with DH 
> using keyring-based key material. The resulting shared secret is again 
> maintained in the key ring where user space can use the symmetric ciphers of 
> the kernel with those keys. User space would only see references to keys but 
> no real keys. However, only user space knows when to invoke what cipher to 
> implement a specific protocol.

You could also keep the secret in a master process and talk to that via
ipc.

Bye,
Hannes