Re: [dm-crypt] LUKS - SSD trim

[Arno Wagner <arno@wagner.name>]

On Sat, Apr 24, 2010 at 12:59:18AM +0200, Richard Zidlicky wrote:
> On Fri, Apr 23, 2010 at 10:45:34PM +0200, Milan Broz wrote:
> 
> > I asked how TRIM (and SCSI discard) is handled and it seems that most of drives zeroes
> > trimmed blocks (or the function is internal to drive - so undefined, we must expect the worst case).
> > 
> > So it is clear that an attacker can recognize which block was trimmed (reading
> > zeroed blocks instead of "random" data). If there is some internal drive data
> > related to TRIM available, it can be even worse.
> 
> it can be even much worse. The most evil case is a specially crafted device manufactured
> by a mighty agency which will record every single read/write/trim command. A few weeks ago
> it was on the news here that most copiers have builtin hard discs and make copies of what
> people are copying.. so who knows what is in our ordinary hard discs today.

Only as temporary storage. Better copiers do a secure erase after
the copy as well, but not all do. It is something copiers used
for secret material can get certified for.
 
> The second most evil - and very realistic scenario is that any drive can
> have a log storing trim states/operations. If its an SSD device that log
> could be *very* huge if we are unlucky.  Although I am not an expert on
> this devices I would think it is the obvious way to do it. All the
> information about trimmed blocks must be stored somewhere and in order to
> keep the "wear" low its likely to be log-structured, not a simple bitmask. 
> There is not an official way to retrieve this backlog but there may be
> device specific undocumented proprietary ways to do it or someone might
> open the chip.
>
> But there sure is no way to guarantee that any information that is ever
> entrusted to a blackbox device might not resurface again.

That is way you use encryoption on top. However, it is higly unlikely
current HDDs/SSDs store a lot of information of this type. The storage
space is just not there.

Arno

-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier