On 21 June 2017 at 14:49, David Howells <dhowells@xxxxxxxxxx> wrote: > 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. > That would be cleaner, yes. As I said, it looks correct to me, but I would simply prefer to keep the code as simple as possible. If there is a good reason for the additional fast path, then I have no objections at all. >> 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). > Ah ok. That does complicate matters, indeed. > (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. > That makes sense, but deserves another big fat warning when it turns out that the blacklist contains an entry we cannot verify against. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
