A complicating factor here is the existence of many layers of abstraction between a high-level request to delete data and what actually happens on the device in question. In between, many things could cause trouble.
- For example, a traditional filesystem usually breaks a file into a series of chunks, each sized to occupy a disk sector, and distributes these chunks on the disk according to various heuristics intended to improve performance. Some type of index table, perhaps in a sector of its own, indicates where each chunk is. In such a system, when the higher-level software deletes or even shrinks a file, the filesystem may respond by clearing that entry in that file's index table and marking that sector as "free." However, the deleted data may remain on the disk, in this now-free sector. (Issues such as this led to the object reuse worries of the Orange Book world of Chapter 2.)
- Journaling filesystems, a more advanced technology, make things even worse. Journaling filesystems treat the disk not as a place to store files so much as a place to store a log of changes to files. As with Word's Fast Save option (see Chapter 13), the history of edits that resulted in a file's current state may be available to the adversary inspecting the disk itself.
- Computing hardware has seen a sort of trickle-down (or perhaps smarting-down) effect, whereby traditionally "dumb" peripherals now feature their own processors and computing ability. Disk controllers are no exception to this trend, leading to yet another level of abstraction between the view the computing system sees and what actually happens with the physical media.
In recent years, semiconductor FLASH memory (e.g., in USB thumbdrives) has probably become more ubiquitous than magnetic media for removable storage. FLASH is also standard nonvolatile storage in most embedded devices, such as cell phones and PDAs. The internal structure of a FLASH device is a bit more complex than other semiconductor memories (e.g., see [Nii95]). FLASH is organized into sectors, each usually on the order of tens or hundreds of kilobytes. When in "read" mode, the device acts as an ordinary ROM. To write a sector, the system must put the FLASH device into write mode, which requires writing a special sequence of bytes, essentially opcodes, to special addresses in the FLASH device. Typically, the stored bits can be written only one way (e.g., change only from 0 to 1). To erase a sector (e.g., clearing all the bits back to 0), another sequence of magic bytes must be written. Often, FLASH devices include the ability to turn a designated sector into ROM by wiring a pin a certain way at manufacture time.
These technical limitations lead to incredible acrobatics when designing a filesystem for FLASH (e.g., [GT05, Nii95]). In order to avoid wearing out the FLASH sectors, designers will use data structures that selectively mark bits to indicate dirty bytes within sectors and rotate usage throughout the sectors on the device. For fault tolerance, designers may try to make writes easy to undo, so that the old version of a file can be recovered if a failure occurs during the nontrivial duration of a write. Even relatively simple concepts, such as a directory or index table, get interesting—if you decide to keep one, then you'll quickly wear out that sector, even if you're clever with the rest of the files.
FLASH architecture has several consequences for security.
- Because of these log-structured and fault-tolerant contortions, old data may still exist in the device even if the higher levels of the system thought it was erased.
- Because an error in a product's ROM can be expensive, at least one vendor includes an undocumented feature to rewrite the ROM sector by writing a magic series of bytes to the chip. (The complexity of the legitimate magic-byte interface makes it hard to otherwise discover such back doors.)
- Because of the large market demand for low-cost thumbdrives and the smarting down of computation into peripherals, much engineering has gone into commercial FLASH drives, leading to a gap between even the API the encapsulated device provides and the internal state.
Read the rest of Chapter 16: Hardware-Based Security.
Reproduced from the book The Craft of System Security Copyright , Addison Wesley Professional. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other users.