On Thu, Aug 08, 2019 at 07:21:39PM +0800, Gao Xiang wrote: > On Thu, Aug 08, 2019 at 05:29:47PM +0800, Gao Xiang wrote: > > On Thu, Aug 08, 2019 at 06:16:47PM +1000, Dave Chinner wrote: > > > On Wed, Aug 07, 2019 at 10:49:36PM -0700, Eric Biggers wrote: > > > > FWIW, the only order that actually makes sense is decrypt->decompress->verity. > > > > > > *nod* > > > > > > Especially once we get the inline encryption support for fscrypt so > > > the storage layer can offload the encrypt/decrypt to hardware via > > > the bio containing plaintext. That pretty much forces fscrypt to be > > > the lowest layer of the filesystem transformation stack. This > > > hardware offload capability also places lots of limits on what you > > > can do with block-based verity layers below the filesystem. e.g. > > > using dm-verity when you don't know if there's hardware encryption > > > below or software encryption on top becomes problematic... > > ...and I'm not talking of fs-verity, I personally think fs-verity > is great. I am only talking about a generic stuff. > > In order to know which level becomes problematic, there even could > be another choice "decrypt->verity1->decompress->verity2" for such > requirement (assuming verity1/2 themselves are absolutely bug-free), > verity1 can be a strong merkle tree and verity2 is a weak form (just > like a simple Adler-32/crc32 in compressed block), thus we can locate > whether it's a decrypt or decompress bug. > > Many compression algorithm containers already have such a weak > form such as gzip algorithm, so there is no need to add such > an extra step to postprocess. > > and I have no idea which (decrypt->verity1->decompress->verity2 or > decrypt->decompress->verity) is faster since verity2 is rather simple. > However, if we use the only strong form in the end, there could be > a lot of extra IO and expensive multiple-level computations if files > are highly compressible. > > On the other hand, such verity2 can be computed offline / avoided > by fuzzer tools for read-only scenerios (for example, after building > these images and do a full image verification with the given kernel) > in order to make sure its stability (In any case, I'm talking about > how to make those algorithms bug-free). > > All I want to say is I think "decrypt->verity->decompress" is > reasonable as well. ... And another fundamental concern is that if we don't verify earlier (I mean on-disk data), then untrusted data will be transformed (decompressed and even decrypted if no inline encryption) with risk, and it seems _vulnerable_ if such decrypt / decompress algorithms have _security issues_ (such as Buffer Overflow). It seems that it's less security than do verity earlier. Thanks, Gao Xiang > > Thanks, > Gao Xiang > > > > > Add a word, I was just talking benefits between "decrypt->decompress-> > > verity" and "decrypt->verity->decompress", I think both forms are > > compatible with inline en/decryption. I don't care which level > > "decrypt" is at... But maybe some user cares. Am I missing something? > > > > Thanks, > > Gao Xiang > >
