secure erase data partitio

by William Enck » Mon, 12 Apr 2010 19:45:27 GMT

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 Does anyone know if the "Factory defaults" option in Android's recovery mode 
performs a secure erase of the data partition, or is it just a "quick format"? 
I'm not worried about a multiple pass overwrite, I just want to know if the 
partition is completely zero'd out. This should be sufficient for most users 
(since it is a NAND and not a magnetic disk --- and some say a zero pass is 
sufficient for magnetic disks).

Nothing obvious turned up in a quick web search (maybe I wasn't looking hard 
enough), so I took a look at what's going on.

Here's what I found:

- comments at the top indicate the factory reset option calls erase_root() on 
- erase_root() calls format_root_device() in roots.c

- format_root_device() gets the MTD context "write" and calls 
mtd_erase_blocks(write, -1)

- mtd_erase_blocks(MtdWriteContext *ctx, int blocks) appears to determine the 
total number of blocks if a negative value is passed.

It appears that, yes, Android zeros the entire /data partition.

Do others agree that this is what is happening? (and hopefully this thread 
shows up in search results for future references).


William Enck
PhD Candidate
Department of Computer Science and Engineering
The Pennsylvania State University

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