Re: [RFC PATCH 1/8] fscrypt: add fscrypt_prepare_new_inode() and fscrypt_set_context()

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2020-08-24 at 11:21 -0700, Eric Biggers wrote:
> On Mon, Aug 24, 2020 at 12:48:48PM -0400, Jeff Layton wrote:
> > > +void fscrypt_hash_inode_number(struct fscrypt_info *ci,
> > > +			       const struct fscrypt_master_key *mk)
> > > +{
> > > +	WARN_ON(ci->ci_inode->i_ino == 0);
> > > +	WARN_ON(!mk->mk_ino_hash_key_initialized);
> > > +
> > > +	ci->ci_hashed_ino = (u32)siphash_1u64(ci->ci_inode->i_ino,
> > > +					      &mk->mk_ino_hash_key);
> > 
> > i_ino is an unsigned long. Will this produce a consistent results on
> > arches with 32 and 64 bit long values? I think it'd be nice to ensure
> > that we can access an encrypted directory created on a 32-bit host from
> > (e.g.) a 64-bit host.
> 
> The result is the same regardless of word size and endianness.
> siphash_1u64(v, k) is equivalent to:
> 
> 	__le64 x = cpu_to_le64(v);
> 	siphash(&x, 8, k);
> 

In the case where you have an (on-storage) inode number that is larger
than 2^32, x will almost certainly be different on a 32 vs. 64-bit
wordsize.

On the box with the 32-bit wordsize, you'll end up promoting i_ino to a
64-bit word and the upper 32 bits will be zeroed out. So it seems like
this means that if you're using inline hardware you're going to end up
with a result that won't work correctly across different wordsizes.

Maybe that's ok, but it seems like something that could be handled by
hashing a different value.

> > It may be better to base this on something besides i_ino
> 
> This code that hashes the inode number is only used when userspace used
> FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32 for the directory.  IV_INO_LBLK_32 modifies
> the encryption to be optimized for eMMC inline encryption hardware.  For more
> details, see commit e3b1078bedd3 which added this feature.
> 
> We actually could have hashed the file nonce instead of the inode number.  But I
> wanted to make the eMMC-optimized format similar to IV_INO_LBLK_64, which is the
> format optimized for UFS inline encryption hardware.
> 
> Both of these flags have very specific use cases; they make it feasible to use
> inline encryption hardware
> (https://www.kernel.org/doc/html/latest/block/inline-encryption.html)
> that only supports a small number of keyslots and that limits the IV length.
> 
> You don't need to worry about these flags at all for ceph, since there won't be
> any use case to use them on ceph, and ceph won't be declaring support for them.

Ahh, good to know. Thanks!
-- 
Jeff Layton <jlayton@kernel.org>