[149342] in cryptography@c2.net mail archive
Re: [Cryptography] cheap sources of entropy
daemon@ATHENA.MIT.EDU (Jerry Leichter)
Mon Feb 3 19:38:39 2014
X-Original-To: cryptography@metzdowd.com
From: Jerry Leichter <leichter@lrw.com>
In-Reply-To: <7A97CB60-3422-405A-A5F5-DC7D6E1B18C5@gmail.com>
Date: Mon, 3 Feb 2014 18:12:40 -0500
To: John Kelsey <crypto.jmk@gmail.com>
Cc: Bill Stewart <bill.stewart@pobox.com>,
"cryptography@metzdowd.com" <cryptography@metzdowd.com>,
John Gilmore <gnu@toad.com>, "James A. Donald" <Jamesd@echeque.com>
Errors-To: cryptography-bounces+crypto.discuss=bloom-picayune.mit.edu@metzdowd.com
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On Feb 3, 2014, at 12:14 PM, John Kelsey <crypto.jmk@gmail.com> wrote:
>> So, if an attacker running malware in a hypervisor (or SMM) knew you
>> were depending on disk drive timings for the random numbers that
>> create your encryption keys, how easily could they attack you by
>> rigidizing those interrupt timings, e.g. delaying your virtual =
machine
>> interrupts at to the next even 1/60th of a second?
> Maybe this is just my lack of understanding coming out, but I'm having =
a hard time seeing how any crypto code is going to remain secure if the =
hypervisor controlling the VM it's running on is under an attacker's =
control. =20
No, you've got it right. A hypervisor has complete control over =
everything the guest OS can see. It controls the horizontal; it =
controls the vertical. :-) While a hypervisor could not realistic =
monitor every instruction a guest executes - the slowdown would be =
recognized rapidly - it does have control over every interaction the =
guest has with the outside world. And that includes, of course, program =
loading, paging, and so on.
In general*, every layer of the abstraction stack can completely subvert =
every layer above it. A user-mode process is completely at the mercy of =
the OS; the OS is completely at the mercy of the hypervisor; the =
hypervisor is completely at the mercy of the microcode; the microcode is =
completely at the mercy of the hardware. (Everything above the =
microcode is also completely at the mercy of System Management Mode, =
which doesn't fit exactly into this hierarchy - but in some sense is =
really "below" the hypervisor. Similarly, you have to consider attacks =
on pieces of hardware other than the CPU - see the paper recently =
referred to here showing how to "spike" disk drive code.)
*The "in general" isn't quite the case. There's some very interesting =
recent work on how to use the virtualization hardware to protect =
processes from malicious OS's. Very clever stuff. Basically, you =
arrange that any given page is set in the hardware so that the process =
XOR the OS has access to it. When the OS needs access, you encrypt and =
MAC the page before resetting the page tables; when the process needs =
access back, you check the MAC and decrypt. You similarly protect =
anything that gets written to disk. So the OS can manage the processes =
- create them, give them memory, map code into them - but it can neither =
read nor modify the code that gets run or anything that code actually =
does in memory. (There are tons more details. I can't immediately lay =
my hands on the paper I read, but =
https://www.usenix.org/legacy/events/hotsec08/tech/full_papers/ports/ports=
_html/is an overview from some members of the group working on this.)
The reason I mention this is to raise the question: Can similar ideas =
be applied at other levels of the abstraction hierarchy? Until I saw =
these papers, I would have dismissed the idea that one could protect a =
process from a malicious OS. Granted, you now have to trust the =
virtualization hardware and the protection code that uses it. But (a) =
you always end up trusting *something*; (b) the pieces you have to trust =
are much smaller (hence much mor amenable to verification) than a full =
OS.
-- Jerry
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