Re: [PATCH RERESEND v9 0/9] fs: interface for directly reading/writing compressed data

[Eric Biggers <ebiggers@kernel.org>]

On Mon, May 17, 2021 at 03:27:48PM -0700, Omar Sandoval wrote:
> On Mon, May 17, 2021 at 02:32:47PM -0700, Linus Torvalds wrote:
> > On Mon, May 17, 2021 at 11:35 AM Omar Sandoval <osandov@osandov.com> wrote:
> > >
> > > Patches 1-3 add the VFS support, UAPI, and documentation. Patches 4-7
> > > are Btrfs prep patches. Patch 8 adds Btrfs encoded read support and
> > > patch 9 adds Btrfs encoded write support.
> > 
> > I don't love the RWF_ENCODED flag, but if that's the way people think
> > this should be done, as a model this looks reasonable to me.
> > 
> > I'm not sure what the deal with the encryption metadata is. I realize
> > there is currently only one encryption type ("none") in this series,
> > but it's not clear how any other encryption type would actually ever
> > be described. It's not like you can pass in the key (well, I guess
> > passing in the key would be fine, but passing it back out certainly
> > would not be).  A key ID from a keyring?
> > 
> > So there's presumably some future plan for it, but it would be good to
> > verify that that plan makes sense..
> 
> What I'm imagining for fscrypt is:
> 
> 1. Add ENCODED_IOV_ENCRYPTION_* types for fscrypt. Consumers at least
>    need to be able to distinguish between encryption policy versions,
>    DIRECT_KEY policies, and IV_INO_LBLK_{64,32} policies, and maybe
>    other details.
> 2. Use RWF_ENCODED only for the data itself.
> 3. Add new fscrypt ioctls to get and set the encryption key.
> 
> The interesting part is (3). If I'm reading the fscrypt documentation
> correctly, in the default mode, each file is encrypted with a per-file
> key that is a function of the master key for the directory tree and a
> per-file nonce.
> 
> Userspace manages the master key, we have a FS_IOC_GET_ENCRYPTION_NONCE
> ioctl, and the key derivation function is documented. So, userspace
> already has all of the pieces it needs to get the encryption key, and
> all of the information it needs to decrypt the data it gets from
> RWF_ENCODED if it so desires.
> 
> On the set/write side, the user can set the same master key and policy
> with FS_IOC_SET_ENCRYPTION_POLICY, and we'd need something like an
> FS_IOC_SET_ENCRYPTION_NONCE ioctl (possibly with a requirement that it
> be set when the file is empty). I think that's it.
> 
> The details will vary for the other fscrypt policies, but that's the
> gist of it. I added the fscrypt maintainers to correct me if I missed
> something.
> 

Well, assuming we're talking about regular files only (so file contents
encryption, not filenames encryption), with fscrypt the information needed to
understand a file's encrypted data is the following:

1. The encryption key

2. The filesystem's block size

3. The encryption context:

    struct fscrypt_context_v2 {                                                      
         u8 version; /* FSCRYPT_CONTEXT_V2 */                                     
         u8 contents_encryption_mode;                                             
         u8 filenames_encryption_mode;                                            
         u8 flags;                                                                
         u8 __reserved[4];                                                        
         u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];                   
         u8 nonce[FSCRYPT_FILE_NONCE_SIZE];                                       
    };                                                                               

   (Or alternatively struct fscrypt_policy_v2 + the nonce field separately;
    that results in the same fields as struct fscrypt_context_v2.)

This is definitely more complex than the compression cases like "the data is a
zlib stream".  So the question is, how much of this metadata (if any) should
actually be passed around during RWF_ENCODED pread/pwrite operations, and how
much should be out-of-band.

I feel like this should be mostly out-of-band (e.g. via the existing ioctls
FS_IOC_{GET,SET}_ENCRYPTION_POLICY), especially given that compression and
encryption could be combined which would make describing the on-disk data even
more difficult.

But I'm not sure what you intended.

- Eric