Re: [PATCH] crypto: expose needs_key in procfs

[Christoph Böhmwalder]

On 01.03.21 19:47, Eric Biggers wrote:
> On Mon, Mar 01, 2021 at 05:59:17PM +0100, Christoph Böhmwalder wrote:
> > Currently, it is not apparent for userspace users which hash algorithms
> > require a key and which don't. We have /proc/crypto, so add a field
> > with this information there.
> >
> > Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@xxxxxxxxxx>
> >
> > ---
> >   crypto/shash.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/crypto/shash.c b/crypto/shash.c
> > index 2e3433ad9762..d3127a0618f2 100644
> > --- a/crypto/shash.c
> > +++ b/crypto/shash.c
> > @@ -477,6 +477,9 @@ static void crypto_shash_show(struct seq_file *m, struct crypto_alg *alg)
> >   	seq_printf(m, "type         : shash\n");
> >   	seq_printf(m, "blocksize    : %u\n", alg->cra_blocksize);
> >   	seq_printf(m, "digestsize   : %u\n", salg->digestsize);
> > +	seq_printf(m, "needs key    : %s\n",
> > +		   crypto_shash_alg_needs_key(salg) ?
> > +		   "yes" : "no");
> >   }
> >   
>
> Do you have a specific use case in mind for this information?  Normally, users
> should already know which algorithm they want to use (or set of algorithms they
> might want to use).

I have a pretty specific use case in mind, yes. For DRBD, we use crypto 
algorithms for peer authentication and for the online-verify mechanism 
(to verify data integrity). The peer authentication algos require a 
shared secret (HMAC), while the verify algorithms are just hash 
functions without keys (we don't configure a shared secret here, so 
these must explicitly be "keyless").

Now, we also have a solution which sits on top of DRBD (LINSTOR), which 
resides purely in userspace. We recently implemented a feature where 
LINSTOR automatically chooses the "best" verify algorithm for all nodes 
in a cluster. It does this by parsing /proc/crypto and prioritizing 
accordingly. The problem is that /proc/crypto currently doesn't contain 
information about whether or not an algorithm requires a key – i.e. 
whether or not it is suitable for DRBD's online-verify mechanism.

See this commit for some context:
https://github.com/LINBIT/drbd/commit/34ee32e6922994c8e9390859e1790ca

> Also, what about algorithms of type "ahash"?  Shouldn't this field be added for
> them too?

You're right. Since we only work with shash in DRBD, I blindly only 
considered this. I will add the field for ahash too.

> Also, what about algorithms such as blake2b-256 which optionally take a key (as
> indicated by CRYPTO_ALG_OPTIONAL_KEY being set)?  So it's not really "yes" or
> "no"; there is a third state as well.

Correct me if I'm missing something, but crypto_shash_alg_needs_key reads:

static inline bool crypto_shash_alg_needs_key(struct shash_alg *alg)
{
	return crypto_shash_alg_has_setkey(alg) &&
		!(alg->base.cra_flags & CRYPTO_ALG_OPTIONAL_KEY);
}

So this already accounts for optional keys. It just returns "no" for an 
optional key, which seems like reasonable behavior to me (it doesn't 
*need* a key after all).

Another option would be to make it "yes/no/optional". I'm not sure if 
that's more desirable for most people.

> - Eric
Thanks,
--
Christoph Böhmwalder
LINBIT | Keeping the Digital World Running
DRBD HA —  Disaster Recovery — Software defined Storage