On Sun, 2025-03-23 at 15:09 +0100, Nicolai Stange wrote: > Normally IMA would extend a template hash of each bank's associated > algorithm into a PCR. However, if a bank's hash algorithm is > unavailable to the kernel at IMA init time, it would fallback to > extending padded SHA1 hashes instead. > > That is, if e.g. SHA-256 was missing at IMA init, it would extend > padded SHA1 template hashes into a PCR's SHA-256 bank. > > The ima_measurement command (marked as experimental) from ima-evm- > utils would accordingly try both variants when attempting to verify a > measurement list against PCRs. keylime OTOH doesn't seem to -- it > expects the template hash type to match the PCR bank algorithm. I > would argue that for the latter case, the fallback scheme could > potentially cause hard to debug verification failures. > > There's another problem with the fallback scheme: right now, SHA-1 > availability is a hard requirement for IMA, and it would be good for > a number of reasons to get rid of that. However, if SHA-1 is not > available to the kernel, it can hardly provide padded SHA-1 template > hashes for PCR banks with unsupported algos.
I think this was done against the day IMA only supported sha1 and the TPM sha256 and beyond so there'd at least be a record that could be replayed. I think today with most distros defaulting IMAs hash to sha256 that's much less of a problem. > There are several more or less reasonable alternatives possible, > among them are: > a.) Instead of padded SHA-1, use padded/truncated ima_hash template > hashes. > b.) Don't extend unsupported banks at all. > c.) Record every event as a violation, i.e. extend unsupported banks > with 0xffs. > d.) Invalidate unsupported banks at least once by extending with a > unique > constant (e.g. with 0xfes). Instead of any of that, why not do what the TCG tells us to do for unsupported banks and simply cap them with 0xffffffff record EV_SEPARATOR and stop extending to them? (note this would probably require defining a separator event for IMA) Regards, James
