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SubjectRe: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack
On Mon, Dec 22, 2014 at 3:23 PM, Hector Marco Gisbert <> wrote:
>> Before I even *consider* the code, I want to know two things:
> >
> > 1. Is there actually a problem in the first place? The vdso
> > randomization in all released kernels is blatantly buggy, but it's
> > fixed in -tip, so it should be fixed by the time that 3.19-rc2 comes
> > out, and the fix is marked for -stable. Can you try a fixed kernel:
> >
> >
> Well if it is already fixed, then great!.
> But since the vdso is something like a library (cause it contains code,
> and no data), it shall be handled as a library and so it shall be
> located jointly with the other libraries rather than close to the stack.
> Later I'll talk about randomizing libraries among them.
> I think that the core idea of the current ASLR implementation is that
> all the areas that share similar content (libraries, stack, heap,
> application) shall be placed together. Following more or less the MILS
> division. This way, a memory leak of an address of the stack is not very
> useful for building a ROP on the libraries.
> Another issue is the page table locality. The implementation tries to
> allocate the vdso "close" to the stack so that is fits into the PMD of
> the stack (and so, use less pages for the pagetables). Well, placing the
> vdso in the mmap area would solve the problem at once.
> Unfortunately, with your path the VDSO entropy has 18 entropy bits. But
> this is not true. The real entropy is masked with the entropy of the
> stack. In other words, if an attacker guesses where the stack is placed
> they have to do negligible work to guess where the VDSO is located.
> Note that, a memory leak from a data area (which is of little help to
> the attacker) can be used to locate the VDSO (which is of great interest
> because it is executable and contains nice stuff).

I'm not sure it's negligible. It's 9 bits if the attacker can figure
out the stack alignment and 18 bits if the attacker can't. This isn't
great, but it's far from nothing.

> Using my solution, the VDSO will have the same 28 bits of randomness
> than the libraries (but all will be together).
> After after 10000 executions I have found 76 repeated addresses (still
> low entropy, but much better than before). But with my patch, there was
> no repetition (much better entropy).
> > 2. I'm not sure your patch helpes. The currently exciting articles on
> > ASLR weaknesses seem to focus on two narrow issues:
> >
> > a. With PIE executables, the offset from the executable to the
> > libraries is constant. This is unfortunate when your threat model
> > allows you to learn the executable base address and all your gadgets
> > are in shared libraries.
> Regardes the offset2lib... The core idea is that we shall consider the
> application code and libraries as two slightly different things (or two
> different security regions). Since applications are in general more
> prone to have bugs than libraries, it seems that this is the way to do
> it from the security point of view. Obviously, stack and libraries are
> clearly apart (you can even assign different access permissions).
> Application code and libraries are not that different, but it would be
> better of they are not together.... and sincerely, I think that the cost
> of allocate them apart is so small that it worth the code.
> If the extra cost of (One or two pages) per process required to place
> the application code to another area is too high, then may be it can be
> implemented as another level of ASLR randomize_va_space=3 (if any).
> > b. The VDSO base address is pathetically low on min entropy. This
> > will be dramatically improved shortly.
> >
> > The pax tests seem to completely ignore the joint distribution of the
> > relevant addresses. My crystal ball predicts that, if I apply your
> > patch, someone will write an article observing that the libc-to-vdso
> > offset is constant or, OMG!, the PIE-executable-to-vdso offset is
> > constant.
> >
> > So... is there a problem in the first place, and is the situation
> > really improved with your patch?
> >
> > --Andy
> Absolutely agree.
> The offset2x shall be considered now. And rather than moving objects
> like the vdso, vvar stack, heap... etc.. etc.. we shall consider
> seriously the cost of a full (all maps) to be real random. That is
> inter-mmap ASLR.
> Current implementation is not that bad, except that the application was
> considered in the same "category" than libraries. But I guess that it
> deserves a region for its own. Also, I think that executable code shall
> be apart from data.. which supports the idea of inter-mmap randomization.
> Sorry if I'm mixing VDSO, and offset2lib issues, but they share a
> similar core problem.

If I see a real argument that randomizing the vdso like a library is
better than randomizing it separately but weakly, I'll gladly consider
it. But the references I've seen (and I haven't looked all that hard,
and I'm not an memory exploit writer) are unconvincing.

I'd really rather see a strong inter-mmap randomization scheme adopted.


> --Hector Marco.

Andy Lutomirski
AMA Capital Management, LLC

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