On Fri, Sep 08, 2017 at 02:48:51PM +1000, James Morris wrote:

Mimi and Christoph worked together on this over several iterations -- I'll 
let them respond.

Mimi --- we should chat next week in LA.  I've been working on a
design internally at work which proposes a generic VFS-layer library
(ala how fscrypt in fs/crypto works) which provides data integrity
using per-file Merkle trees.

The goals of this design:

   * Simplicity; for ease in security review and upstream review and
     acceptance
   * Useful for multiple use cases.  It is *not* Android/APK specific,
     and indeed can be used for other things
      * A better way of providing Linux IMA/EVM support for immutable
        files by moving the verification from time-of-open to
        time-of-readpage.  (This significantly reduces the performance
	impact, since we don't need to lock down the file while the kernel
	needs to run SHA1 on potentially gigabytes worth of file data.)
      * Most use cases for file-level checksums are for files that
        don?t change over time (e.g., for Video, Audio, Backup files,
        etc.)  This allows us to provide a cheap and efficient way to
        provide checksum protect against storage-level corruption
        fairly easily.  So by supporting both SHA and CRC-32, we can
	make this feature useful for more than just the security heads.  :-)
   * Like the encryption/fscrypt feature, most of the code to this
     feature can be in a VFS-level library, with minimal hooks needed
     to those file systems (ext4, f2fs) that wish to provide this
     functionality.

					- Ted
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