[E-Lang] defense in depth

Mark S. Miller markm@caplet.com
Sun, 28 Jan 2001 23:44:28 -0800


At 01:49 PM Wednesday 1/24/01, Nikita Borisov wrote:
>Of course, this additional layer does not come without cost.  But the
>question is to trade off this cost of defense in depth against the
>potential benefits.  It seems that a lot of people in this thread are
>asserting that the additional benefit is zero, which I find hard to
>believe.  Am I misunderstanding what people are saying?

I can only speak definitively for myself, though I do believe Tyler 
especially is saying compatible things.  I think Tyler & I both tried to 
make a distinction between two kinds of redundancy that aid security, but 
I'm not sure if this distinction took.  Both sides of this distinction might 
be called "defense in depth", so, to avoid confusion, I'll use two different 
names: "redundant resistance" and "debugging".


* Redundant resistance:  On the one hand, we have defenses in the physical 
world against physical attack, in which quantitative degrees of attack are 
matched against quantitative degrees of defense, and an overwhelming 
quantity on either side often leads to success.  In this world, redundant 
defenses are sensible precautions against ever greater efforts by an attacker.

In computer security, I said these kinds of defenses are sensible regarding 
the embedding of computation in the physical and social world (social 
engineering attacks, microwave attacks, power analysis, etc), and regarding 
the faithfulness of the realization of our security primitives out of more 
primitive material (cryptanalysis).

In this thread, when people used the phrase "defense in depth", I've been 
taking them to mean this case (redundant resistance) specifically.  As this 
thread has developed, I realize I may have interpreted them wrongly.  If so, 
apologies for the confusion.


* Debugging: OTOH, we have defenses within the logical/mathematical world in 
which those security primitives are the "laws of physics".  In this world, 
our vulnerability to attack is almost never due to greater efforts on the 
part of an attacker, but rather to a bug in our own defensive logic.  
Redundancy at this level should be targeted at catching us in our own 
stupidity, not at containing the damage from an attacker that has mounted a 
heroic effort and overcome our first line of defense.  The "rely" and 
"suspect" notation I proposed is precisely an example of such redundancy for 
debugging, as are static type systems in normal software engineering.

A note on perfection: I never claimed that perfection was actually possible. 
Rather, the danger to ourselves is our shortfall from perfection, and 
therefore it's better to spend the effort debugging (looking and fixing 
logical flaws) rather than piling up logically flawed defenses on one 
another.  This corresponds directly to conventional wisdom in non-security 
software engineering.  When one reads code and sees some parts of a 
program whose purpose is to compensate for bugs in other parts of a program, 
this isn't normally taken to be a sign of robustness.  Robust code gets that 
way, not by compensating for bugs in the hope of surviving them, but by 
making bugs cause a clear and diagnostic failure, so they get fixed early.


                            My Retraction


However, in light of MarcS' posts, I now realize that my main defensive 
tool, the Principle of Least Authority (POLA) (also known as the Principle 
of Least Privilege) fits in both the above categories.  It fits in the 
debugging category because it vastly aids our informal proofs-in-the-head 
when reasoning about dangers.  It so radically bounds the dangers left to 
consider in examining a particular body of code as to make the examination 
vastly more tractable, and vastly more likely to spot actual 
vulnerabilities.  Not perfect, but a huge help.

But, MarcS also makes a good point when he observes that, in a system built 
according to a strict and fine grained application of POLA, an attacker that 
get past one set of defenses (because of uncaught security bugs, not because 
of greater attacker effort) will *often* (not always) find there's only very 
limited authority for him to abuse, and further locked doors between him and 
more authority.  POLA also leads to redundant resistance.  My retraction: 
I think this is good.

(Note: There are also many places where there's only one wall between an 
attacker and the ability to be all powerful, such as Norm's example of the 
ability to take control of the MMU.  For these, I continue to believe the 
correct answers are careful examination, debugging, and redundancy only in 
support of debugging, not redundant resistance.)

The KeyKOS inspired systems (KeyKOS, EROS, Joule, E) all use POLA both to 
aid bug finding, and as redundancy in resisting an attacker.  Despite having 
retracted my earlier claim, I still find no criticism of the systems from 
which I abstracted this claim.  Rather, I find I understand the strength of 
these systems better.  My respect has only gone up for them, for POLA, and 
yes, for redundant resistance.

Having conceded the value of redundant resistance, I reluctantly concede 
that there is, in theory at least, as David suggests, a possible tradeoff 
between techniques that aid in debugging's quest for logical perfection, and 
techniques for redundant resistance.  POLA doesn't force us to consider this 
tradeoff, as it aids both.  Should we find a genuine tradeoff, it'll be 
interesting to examine it.  My strong suspicion: it's never good to retreat 
in the quest for logical perfection, as (within this mathematical world) our 
own confusion is always a greater danger than our attacker's efforts.  In 
the systems that inspire the greatest confidence in me and in many others I 
respect, I see no examples of such a retreat.  I look forward to examining 
some concrete proposed tradeoffs.


With these distinctions in hand, would anyone like to try restating what 
kind of defense stack introspection is supposed to be?



        Cheers,
        --MarkM