linux-ext4 - Re: [RFC PATCH 1/8] fscrypt: add fscrypt_prepare_new_inode() and fscrypt_set

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Date:   Mon, 24 Aug 2020 12:02:21 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, ceph-devel@...r.kernel.org
Subject: Re: [RFC PATCH 1/8] fscrypt: add fscrypt_prepare_new_inode() and
 fscrypt_set_context()

On Mon, Aug 24, 2020 at 02:47:07PM -0400, Jeff Layton wrote:
> 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.

That's only possible if the VFS is truncating the inode number, which would also
break userspace in lots of ways like making applications think that files are
hard-linked together when they aren't.  Also, IV_INO_LBLK_64 would break.

The correct fix for that would be to make inode::i_ino 64-bit.

Note that ext4 and f2fs (currently the only filesystems that support the
IV_INO_LBLK_* flags) only support 32-bit inode numbers.

- Eric