From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Howells Subject: Re: Problem with new X.509 is_hash_blacklisted() interface Date: Wed, 21 Jun 2017 13:49:09 +0100 Message-ID: <8099.1498049349@warthog.procyon.org.uk> References: <1495897525.3458.7.camel@HansenPartnership.com> <26151.1497974983@warthog.procyon.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Return-path: In-Reply-To: Content-ID: <8098.1498049349.1@warthog.procyon.org.uk> Sender: owner-linux-security-module@vger.kernel.org To: Ard Biesheuvel Cc: dhowells@redhat.com, James Bottomley , keyrings@vger.kernel.org, linux-security-module , "linux-efi@vger.kernel.org" , linux-kernel List-Id: linux-efi@vger.kernel.org Ard Biesheuvel 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. David