Re: Problem with new X.509 is_hash_blacklisted() interface

From: David Howells
Date: Wed Jun 21 2017 - 08:49:30 EST

Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:

> > This can be told to skip a particular algorithm for when the caller
> > has one precalculated. The precalculated hash can be passed to
> > is_hash_blacklisted(). This would typically be the case for a signed
> > X.509 message.
> This last part seems a premature optimization to me. Is there a
> performance concern preventing us from using (4) only?

Crypto stuff is relatively slow - and in the case of X.509 and PKCS#7 the
caller will already have calculated a hash. The most likely situation
currently, I think, is that we will only have sha256 hashes in the blacklist,
and whatever we're checking will have a sha256 hash also.

Possibly, I could just pass the precalculated hash into is_data_blacklisted()
and so avoid having to call is_hash_blacklisted() from outside.

> In any case, the approach and the code look sound to me, although I
> think adding a hash of a type that we don't know how to calculate
> deserves a warning at least.

There are two issues with that:

(1) We don't know what hashes are available without checking to see what
modules are available. However, to do this would involve loading the
hash algorithm module - but we might not be in a position to do this yet
(the blacklist is loaded before we start userspace).

(2) A module implementing a hash algorithm might be blacklisted by the hash
that we've been given to add to the blacklist. I think this is a more
general problem - and might require us to restrict blacklisting to hash
algorithms that are built in.