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Re: PointGuard: It's not the Size of the Buffer, it's the Address
From: Crispin Cowan <crispin () immunix com>
Date: Mon, 18 Aug 2003 22:19:11 -0700

Sorry for the length, but its a long post, and I feel the need to rebut most of this.
pageexec () freemail hu wrote:

Here we go then (all quotes are from your paper).

1. "This key is then never shared with any entity outside the process's
   address space....
  "Thus we cannot identify any feasible means by which the attacker can
   obtain the PointGuard key."

You are wrong (and even self-contradicting) here, in any case, so-called
information leaking can happen without having to corrupt pointers ([1],
[2]). Also, section 3.4.3 sublates the above.

It is true that PointGuard raises new issues with regard to information leakage: before PointGuard, there was not much significance to leaking pointer values from a running process, and so this now becomes a new threat that needs study. At the USENIX conference, it was pointed out that format string bugs can be used to obtain pointer values, which we had not thought of. However, composing PointGuard with FormatGuard somewhat mitigates this problem.

However in the implementation part you talk about only those pointers
that are visible at the C language level whereas we know all too well
that there is more than that (ELF GOT/PLT, saved program counter and
frame pointer, etc). Because of this omission it appears that PG does
not protect these pointers at all even if they have been the primary
targets of address space corruption bugs in the past. Is this really
the case or is the paper missing something?

Yes, PointGuard only protects pointer values generated by code compiled with PointGuard.

What really piqued my interest is that the PLT/GOT are not generated
by the compiler hence the implementation you describe cannot possibly
handle them without changes to the dynamic linker - something you do
not mention at all.

We are modifying the dynamic linker for Immunix. But that kind of hacking isn't worthy of a paper, so we omitted it.

It would also be interesting to know how you can
handle the saved program counter and frame pointer just after the AST
level where as far as i know these entities do not even exist (and
hence cannot be manipulated/controlled there).

As the paper said, we are going to tag the AST expressions so that spills are PG-encrypted, but this is not yet implemented.

3. In section 3.4.1 you say about statically initialized data that:

  "[...] we modify the initialization code emitted by the compiler
   (stuff that runs before main()) to re-initialize statically
   initialized pointers with values encrypted with the current
   process's key."

Can you clarify what initialization code the compiler emits before
main()? As far as i know, on entry only the dynamic linker, library
initialization and some statically linked-in object code (various
crt*.o stuff and what they call) gets to run before main() - none
of this is emitted by the compiler, at least not for each executable
as you made it sound to be.

Yes, the static data initialization is hacked into that code. I don't recall whether it is crt0.o or something else, but it doesn't really matter.

4. As mentioned above, section 3.4.3 admits that there are still ways to
  modify non-encrypted pointers in the current implementation (beyond
  the information leaking attacks i mentioned). To me it also means that
  not all pointer stores/loads are protected but only those visible at
  the C language level (refer to the problematic pointers pointed out in
  2). It also begs the question of what kind of performance impact PG
  will have once all these omissions are rectified (more on your
  performance evaluation below).

The only pointer load/stores that are not encrypted right now are register spills. That is a rare case, so it will not affect performance much.

5. In section 3.4.4 you talk about mixed-mode code (PG vs. non-PG). You
  seem to be focused on marking function parameters for use by PG or
  non-PG code but you do not mention what happens with pointers stored
  in data structures which are used by both kinds of code. Do/can you
  mark such structure members with __std_ptr_mode_on__?

Yes.

Also what happens
  with functions that take format strings and hence accept arguments of
  variable types (i.e., pointers and non-pointers), do you parse such
  format strings and convert the pointer arguments accordingly or do
  you turn off PG altogether for such code?

There is special case handling for varargs.

What happens with system
  calls that take pointers? You mention in the paper that you have not
  created a PG version of glibc, so are all pointers passed to system
  calls unprotected?

That is correct: unencrypted pointers are passed into the kernel. It has to be this way, because the alternatives would be to either have a system-wide single key value (which would persist far too long for such a small key, and be too easy to obtain) or to have the kernel know the key value of all processes and do the mapping for you (which is feasible, but more intrusive than just hacking glibc).

What happens to system calls that do not go through
  glibc (there are applications that do this)?

You would have to modify the source to mark those arguments as cleartext.

  In the same section you all of a sudden introduce the notion of
  'hashed pointers' without explaining what they are and how PG uses
  them. Can you elaborate on this?

It's just a synonym for PG-encrypted pointers.

  Finally i am wondering how you plan to implement pointer mode tracking
  in the compiler, or more precisely, why you have to do it in the compiler
  only and not at runtime (in the latter case you would have to extend the
  pointer representation and open a whole can of worms).

I have no idea what you are talking about.

6. In section 5 you admit that you do not indeed have a PG protected
  glibc and hence heap pointers are not protected at all, this calls
  into question the seriousness of your security and performance
  testing (especially since you compare your results to mature
  solutions which cannot be said of PG yet).

All of the code used in our performance testing was statically linked and compiled with PointGuard to work around the absence of a PG version of glibc, so the performance figures are valid.

  "2. Usefully corrupting a pointer requires pointing it at a
   specific location."

This is false, the hijacked pointer may very well point to a set of
specific values (e.g. any GOT entry that is used later, any member of
a linked list, etc).

Bull: you just specified a specific location that happens to be a range. A very small range in the size of an address space. Unlike PaX/ASLR (which can only jiggle objects a little within a range) PointGuard has complete freedom to randomize all 32 bits of the pointer, so the fact that you can craft an exploit that can only approximately hit a target does not affect PointGuard.

Thanks for bringing up this point, as it highlights something important: PointGuard and address space randomization techniques (PaX/ASLR, the Sekar paper that immediately followed PointGuard at USENIX, and several other re-implementations of PaX/ASLR) are complementary.

   * ASLR techniques defend binaries (good for convenience) and as a
     consequence defend objects that are hard to protect with PointGuard.
   * PointGuard provides better randomization than ASLR, because the
     randomization ranges are much greater.
   * These two techniques compose: you are better off using both PG and
     ASLR. Similar to the way in which you are better off using both
     StackGuard and non-executable stacks (Immunix ships with both
     StackGuarded binaries and a non-executable stack kernel).


  "3. Under PointGuard protection, a pointer cannot be corrupted
   to point to a specific location without knowing the secret key."

This is correct provided the implementation is bug-free - something
that cannot be verified until you actually release PG.

I have no idea what you are talking about. If the pointer is hashed, you *cannot* usefully corrupt it without knowing the secret key. Speculating that any piece of software has bugs without foundation boarders on FUD, but in this case it isn't even possible: an encrypted pointer cannot be modified by a plaintext overflow. A bug that accidentally laid a plaintext pointer would result in a crash when the value is decrypted, and vice versa: the design specifically resists this problem.

  "4. Learning the secret key requires either obtaining the secret
   key directly, or cryptanalysis against a sample pointer value."

These methods are called information leaking as discussed above. The
term 'cryptanalysis' is a bogus term here, as it makes it sound like
an expensive operation whereas all it takes is knowing the valid
pointer value (something an attacker can observe on a test system)
and xor'ing it against the leaked one.

It is none the less cryptanalysis. The paper itself points out that the crypto is weak: the security depends on not leaking ciphertext.

  "6. Obtaining a sample of ciphertext (an encrypted pointer) would
   require either corrupting a pointer precisely (which begs the
   question) or a program that leaks pointer values (which is highly
   unusual)."

The latter claim ("highly unusual") is unsubstantiated, what is the
basis for it? At least neither your paper nor anything you referenced
present research data on this. Also there have been papers published
recently on this very topic ([1] and [2]), so it seems we are just
beginning to see the real nature of information leaking (this has
also been pointed out in the PaX ASLR paper [3]).

As I said above, it's a new area, and needs study. Thanks for the citations.

8. In section 6 you present performance evaluation data. The fundamental
  problem with it is of course that PG has apparently not been finished
  yet (something you do not make clear there), therefore any claims about
  its impact are to be taken with a grain of salt.

PointGuard is at almost exactly the same stage of maturity as StackGuard was when the paper first appeared in January 1998. A whole bunch of system engineering has yet to be done. In the case of StackGuard, overall performance *improved* vs. the results claimed in the paper. Speculate away as to what PointGuard will do when we're done integrating it. On second thought, don't: you've done more than enough flaming speculation today :)

  Third, there is related work ([4] and [5], all of which predates PG
  by years and you failed to reference) that appears to show more real
  performance impact of function pointer encryption (something PG does
  not seem to do yet universally).

That work is in fact based directly on PointGuard, having resulted from this post http://lwn.net/1999/1111/a/stackguard.html

And you're on crack if you think their performance results are more realistic: the only "pointer" they encrypt is the activation return address. *None* of the hard work of weaving pointer encryption into the compiler's type system was done. They published first because we chose deliberately to not publish an empty idea with no implementation.

9. In section 7.1 you say that:

  "A developer can port an application to these safer dialects in a few
   hours or days, where as PointGuard was designed to allow a developer
   to compile & protect millions of lines of code in a few hours or day."

whereas you admit before that PG requires programmer intervention (as it
is not possible to have a pure PG system right now), i doubt a programmer
can compile (port) millions of lines of code in a day.

You are entitled to your opinion on the numbers and magnitudes, but it is inescapable that "porting" to PointGuard is far less work than porting from C to Cyclone or CCured. So what's your point?

10. In section 7.2 you claim that:

  "The main limitation is that this defense can be bypassed, because
   suitable attack payload code (effectively "exec(sh)")) is almost
   always resident in victim program address spaces, and so pointer
   corruption is all that is necessary for the determined attacker
   to succeed."

Where is this "exec(sh)" supposed to be 'almost always'? Can you substantiate
this claim?

It is in glibc, and most programs link to glibc. This is very well known, and I didn't think it needed to be justified.

Next you make certain claims about PaX [6] (please observe the proper
capitalization) without providing any reference to our project - why?

I have been *trying* to properly cite PaX in various papers for at least a year, but you don't make it easy. A web URL is not normally considered a suitable citation. At least publish a Phrack article or something so I can actually cite you. FWIW, I have been repeatedly pointing out PaX to various people who are re-inventing ALSR in various forms, because the research community is unaware of PaX. I dare say that the PointGuard paper will do more to raise PaX visibility in the research community than anything before. That was deliberate, because IMHO PaX is under-exposed: it's good work, and few have heard of it.

You also fail to substantiate your claims about the performance of PaX.
My best guess is that you are probably referring to a very old and long
outdated paper, not the current implementation. For your information,
NOEXEC has no performance impact on alpha, i386 (when SEGMEXEC is used,
which is the default, [12]), parisc, sparc and sparc64 and has a small
impact on ppc. I am curious to learn why you cited this information
when you have already been made aware of the current situation ([13]).

It was hearsay. Publish something, and I'll cite it. Please.

11. In section 7.3 you claim that:

  "PaX also incorporates ASLR (Address Space Layout Randomization) which
   can be viewed as the dual of PointGuard: rather than randomizing
   pointers, ASLR randomizes the location of key memory objects."

This is a false claim, ASLR does the exact same thing to pointers as PG.
Think about it, if you randomize all memory regions, then all pointers
to these regions will necessarily be randomized as well.

Go look up the word "dual": it is a mathematical term. What you're saying is exactly the same as what I am saying.

  "Sekar et al [3] have a new implementation of this concept that
   randomizes more elements of the address spacelayout, which may
   make it harder to bypass than PaX/ASLR."

This is misleading because Address Obfuscation is vulnerable to the exact
same information leaking problem as ASLR or PG, otherwise an attacker has
to guess addresses (if he needs any, that is), there is no (determinisctic)
way around that.

It is *your job*, not mine, to go write a paper explaining how PaX/ASLR is better than Sekar et al. Be sure to point out that PaX/ASLR came first, as that is a strong point in your favor. In the absence of such a paper, I'm having to guess at the differences, in a very small portion of my paper. I vigorously encourage you to go write a real paper and submit it to a strong refereed conference such as USENIX Security. Really, please, go write a real paper, I would love to read it, and would cite it as often as I could. Had you done this two years ago, you would not be having this silly flame war over W^X with Theo.

Crispin

--
Crispin Cowan, Ph.D.           http://immunix.com/~crispin/
Chief Scientist, Immunix       http://immunix.com
           http://www.immunix.com/shop/



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