Re: [ubuntu-hardened] Re: Collecting NX information

Arjan van de Ven
2005-03-29T19:37:06Z

On Tue, 2005-03-29 at 14:07 -0500, John Richard Moser wrote:
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> Arjan van de Ven wrote:
> >>>Hmmmm you either need an executable stack or you don't. Can you explain
> >>>why you think there is a strong advantage for a "neutral" setting on
> >>>this one?
> >>>
> >>
> >>As I said, compatibility mode. The toolchain should not emit
> >>*everything* PT_GNU_STACK and leave it up to you to de-mark it; instead,
> >>everything should be emitted without PT_GNU_STACK set, in which case
> >>violating code would die.
> >
> >
> > actually right now the toolchain marks things automatically correct.
> > If gcc emits a stack trampoline, it gets marked needing executable
> > stack, if gcc can prove it doesn't need executable stack, it gets marked
> > as such as well.
> >
>
> And the toolchain emits a -E library with PT_PAX_FLAGS if there's a
> stack trampoline :) But it's defficient right now, doesn't inherit when
> you link to a library with -E. . . you can fix that right? :)

it's inherited for PT_GNU_STACK though.
Not sure how you implemented PT_PAX_FLAGS, but for PT_GNU_STACK it's
inherited.

> > I *really* don't understand why you want to get away from automatic
> > marking to something manual, which *has* to be more fragile.
> >
>
> /me shrugs. It's a security blanket for him mostly; he fears automagic
> security maintainence.

who is "him" ?

>
> >
> >>Remember also I'm very much against "let the compiler guess if you need
> >>an executable stack"
> >
> >
> > it's not guessing. the *compiler* emits the stack trampoline. So the
> > *compiler* knows that it needs that stack.
> >
>
> With a trampoline, things like Grub and a few libs need PT_GNU_STACK.

sure they do. There's about a handful in an entire distro.

>
> Of course you can't just suddenly say "OH! Well PT_GNU_STACK should do
> this instead!" because you'll break everything.

I'm not a fan of any kind of emutrampoline. At all. But I am open to
others making a different tradeoff; for me the option to have a
trampoline at all is just a bypass of the non-exec stack... legit bypass
one hopes but a bypass regardless. Some time ago we did an eval of how
much stuff would need the emutramp (well or something equivalent) and
the list was so small that I decided that the added risk and complexity
were not worth it and that I rather had those 5 or so apps run with exec
stack.

> > again what does tristate mean.. "I don't know" ? But gcc does know, with
> > very very very few exceptions. Eg old mono is the exception because it
> > didn't do a proper mprotect. Saying "I don't know" doesn't solve you
> > anything, since in the end there needs to be a policy enforced anyway,
> > it's just postponing the inevitable to a point with less knowledge.
> >
>
> Remember I'm also thinking of restricted mprotect() situations as well,
> because I'm trying to get it to the point where one set of markings has
> a predictable effect on any kernel, be it vanilla, pax, or ES.

well that is an entirely independent thing again. Really.
I think mixing all these into one big flag is a mistake.
(And thats a lesson I learned the hard way, but Linus was right; don't
mix independent things into one flag artificially. Extra flags are
cheap. Don't complicate the world for a dozen bytes.)