On Tue, May 19, 2020 at 07:13:21AM -0400, Theodore Y. Ts'o wrote: > On Fri, May 15, 2020 at 01:41:41PM -0700, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > The eMMC inline crypto standard will only specify 32 DUN bits (a.k.a. IV > > bits), unlike UFS's 64. IV_INO_LBLK_64 is therefore not applicable, but > > an encryption format which uses one key per policy and permits the > > moving of encrypted file contents (as f2fs's garbage collector requires) > > is still desirable. > > > > To support such hardware, add a new encryption format IV_INO_LBLK_32 > > that makes the best use of the 32 bits: the IV is set to > > 'SipHash-2-4(inode_number) + file_logical_block_number mod 2^32', where > > the SipHash key is derived from the fscrypt master key. We hash only > > the inode number and not also the block number, because we need to > > maintain contiguity of DUNs to merge bios. > > > > Unlike with IV_INO_LBLK_64, with this format IV reuse is possible; this > > is unavoidable given the size of the DUN. This means this format should > > only be used where the requirements of the first paragraph apply. > > However, the hash spreads out the IVs in the whole usable range, and the > > use of a keyed hash makes it difficult for an attacker to determine > > which files use which IVs. > > > > Besides the above differences, this flag works like IV_INO_LBLK_64 in > > that on ext4 it is only allowed if the stable_inodes feature has been > > enabled to prevent inode numbers and the filesystem UUID from changing. > > > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > > Reviewed-by: Theodore Ts'o <tytso@xxxxxxx> > > I kind of wish we had Kunit tests with test vectors, but that's for > another commit I think. > We do have ciphertext verification tests in xfstests for all the existing fscrypt options. Actually, I had hacked one together for IV_INO_LBLK_32 before sending this patch (https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfstests-dev.git/commit/?id=55153ceee2948269c0359bd97fc0d58a26139c87). I'll be sending it for review after I've looked over it again. Similarly, since earlier this year, we now also have ciphertext verification tests in Android's VTS (Vendor Test Suite) (https://android.googlesource.com/platform/test/vts-testcase/kernel/+/refs/heads/master/encryption/). I'll be adding one for this new flag. These ciphertext verification tests test the round-trip from the key added by userspace to the data on-disk -- even if the data is encrypted by inline crypto hardware rather than the kernel itself. So they're better than Kunit tests. The thing I'm struggling with a bit is actually that when inline crypto is used, IV_INO_LBLK_32 introduces a case where the DUN can wrap from 0xffffffff to 0, and that case is new/special in that blocks can't be merged over that boundary even if they are both logically and physically contiguous. So, we could also use a test that tests doing I/O around this boundary where the DUN wraps around. - Eric
