Confidential Data Not Safe On Solid State Disks

An anonymous reader writes "I always thought that the SSD was a questionable place to store private data. These researchers at UCSD's Non-Volatile Systems Laboratory have torn apart SSDs and have found remnant data even after running several open source and commerical secure erase tools. They've also proposed some changes to SSDs that would make them more secure. Makes you think twice about storing data on SSDs — once you put it on, getting it off isn't so easy."

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Summary

[Orgasmatron]

Block storage devices have more capacity than they report. Magnetic disks keep a small reserve of unallocated blocks as a hedge against blocks that fail in use. SSDs keep a much larger reserve because they can only erase in increments that are relatively large compared to their block size.

If you overwrite a sector on a magnetic disk, you will almost always destroy all traces of the old data. The exception is when the drive thinks the old sector has failed or is about to fail, in which case you get an entirely new sector, and your old data is still (possibly) on the old sector. Attacks using magnetic force microscopes to read data from track fringes were possible a decade ago, but there is no reason to think it is possible on a modern drive.

If you overwrite a sector on a SSD, the SSD gives you a whole new block from a list of free blocks, and adds the address of the old block to the list of deleted blocks. Blocks are moved from the deleted list to the free list when the SSD has some free time, or when one is really needed. There is currently no mechanism to force the SSD to actually erase a sector.

This is all known, and there are mechanisms built into the specs to provide a secure erase. What their research is showing, however, is that these mechanisms don't always work. A number of them are buggy, and at least one just plain lies, claiming to have done the secure erase, but actually just doing the normal pointer update trick just like any other write.