Re: [PATCH v2] modsign: use shred to overwrite the private key before deleting it

[Richard Weinberger <richard@nod.at>]

Am 25.01.2015 um 13:24 schrieb Alexander Holler:
> Am 25.01.2015 um 13:08 schrieb Richard Weinberger:
>> On Sun, Jan 25, 2015 at 12:42 PM, Alexander Holler <holler@ahsoftware.de> wrote:
>>> Now, after I ended up into flaming a lot (sorry again, but this topic made
>>> me angry for so long and I had to spent too much time to get rid of unwanted
>>> content and answering other peoples question in regard to that topic), I
>>> should offer something more useful.
>>>
>>> So I've written down in some short words, how I think it could be done:
>>>
>>> First offer a syscall named sunlink() (or whatever name) which fails if it
>>> can't overwrite or securely trim the contents of a file before deleting it.
>>>
>>> That could be done like this:
>>>
>>> (1) If it's a SSD or MMC without offering "Secure Trim" fail.
>>> (2) If it's a plain FLASH or conventional harddisk where writing a block
>>> means that block will be overwritten or if it's a SSD or MMC with "Secure
>>> Trim) go on with
>>> (3) Identify the blocks which contain the file contents (should be doable by
>>> using the same mechanisms used to read and write a file)
>>> (4) Mark the file as deleted
>>> (5) Overwrite or securely trim blocks which can be deleted completely
>>> (6) Build new blocks for blocks which can only partly deleted because they
>>> contain information still used by the FS or other files
>>> (7) Instruct the FS to us the new blocks instead of the old ones
>>> (8) Overwrite or securely trim the old blocks which previously contained
>>> partly information of other stuff.
>>>
>>> Afterwards use that new syscall in shred.
>>>
>>> Of course, this is just a totally simplified instruction in regard to how
>>> complicated filesystems have become, but I think there isn't any black magic
>>> involved in offering the user a simple way to really delete files.
>>
>> Or add support for the "s" chattr to major filesystems.
>>
> And change the manpage for the 's' attribute to change the "overwriting with zero" with some other wording.
> 
> But thanks for the hint. I wasn't aware of that bit (maybe because it's still useless on most filesystems).
> 
> But the above silly instruction might still help in implementing support for the 's' attribute.
> 
> Also I wonder what happens if you delete a file with such an attribute on e.g. an SSD. I assume the user just gets a false positive that the file is deleted, which isn't much
> different to what nowadays happens and doesn't therefor really help.

The implementation will be challenging. Especially for modern filesytems like btrfs or f2fs which are copy-on-write based.

Thanks,
//richard