Tuesday, December 12, 2017

mac_apt + APFS support

Over the past few months, I've been working at adding APFS support into mac_apt, and its finally here. Version 0.2 of mac_apt is now available with APFS support. It also adds a new plugin to process print jobs, some enhanced functionality in other plugins and several minor bug fixes.

As of now basic APFS support is complete, mac_apt can view and extract any file on the file system, including compressed files. It does not have support for FileVault2 (encryption) and will not handle an encrypted volume. The checkpoint feature in APFS is currently not supported or tested although this may be added later.

This is the first forensic processing tool (in freeware) to support APFS. I believe at this time, Sumuri Recon is the only commercial one. I am unaware of any other that can read APFS.

I would like to thank Kurt-Helge Hansen for publishing the paper detailing APFS internal structure and working. He was also helpful in providing a proof of concept code for the same.

The implementation we've used is based on the APFS template built with kaitai-struct. Kaitai-Struct is a library that makes it easy to define and read C structures. It will generate all the code required to read those structures. For APFS, the kaitai-struct template was developed originally by Jonas Plum (@cugu_pio) & Thomas Tempelmann (@tempelorg) here.

APFS working and implementation

The approach we've taken is to read all inodes and populate a database with this data. This means we have to read the entire filesystem data upfront before we have information to read a single file. It isn't ideal, it practically takes 2-4 min. time to do this on an image having default macOS installation (using my slow regular SATA III external disk over USB3), which is not too bad. But I opted for this path as it is the only solution available for now. Why? The way APFS stores files in its b-tree, they are not sorted by name alphabetically. Instead, a 3 byte hash is computed for each file name and the b-tree maintains nodes sorted by this hash instead. The problem is that this hash algorithm is currently unknown. It may just be some sort of CRC variant or something very different. Until this algorithm is known, we cannot write a native parser that walks the b-tree. Hence the database for now.

The database does offer us several advantages though. For compressed file information, we can pre-process the logical size and save that for quick retrieval. In APFS, a compressed file will have its logical size set to zero in file metadata. To lookup its real size, you have to go read its compressed data header (which may be inline or in a resource fork), parse it and get the uncompressed (logical) size. This often means going out to an extent to read it, which makes it slow. Pre-populating this info in a database makes it much quicker for later analysis.

APFS allows extended attributes to be defined and used just the same as HFS+. This means a file can have extended attributes and those are used to save compression parameters (similar to HFS+). APFS also uses Copy-On-Write, which means if you copy a file, the resulting copy will not duplicate the data on disk. Both inodes (original and copy) will point to the same original extents. Only when the copy is changed will new extents be allocated.

If you are not familiar with APFS, the Disk Info output from mac_apt might look strange to you.

Screenshot - Disk Info data from mac_apt showing same offset & size for all APFS volumes
This may be read as 4 partitions all type APFS having the same exact starting offset and size! The reason for this is that APFS is a little different. It isn't just defining a volume, rather it implements a container which can host several volumes in it! This output is from a default installation of HighSierra, where the Disk partitioning scheme is GPT and it defines 2 partitions as seen in the screenshot below.
Illustration showing APFS container and volumes within the Disk

The APFS container by default does not put a limit on the size or location of those volumes within it (Preboot, Macintosh HD, VM, Recovery). Unlike normal partitions on disk where sectors are allocated for each volume before you can use the volumes, APFS allows all volumes to share a common pool of extents (clusters) and they all report having total free space as the same. But really its shared space, so you cannot sum it up for all volumes. This also means data from all volumes is interspersed and volumes are not contiguous. The design makes sense as the target media is flash memory (SSD), and it will never be contiguous there (as it did on spinning HDDs) because of the way flash memory chips work.

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