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SubjectRe: thoughts on kernel security issues
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Ingo Molnar wrote:
> * John Richard Moser <> wrote:
>>Split-out portions of PaX (and of ES) don't make sense. [...]
> which shows that you dont know the exec-shield patch at all, nor those
> split-out portions. At which point it becomes pretty pointless to
> discuss any technical details, dont you think?

I'm shoddy on ES details, but I was more remarking on the overlapping
functions between PaX and ES.

>>PT_GNU_STACK annoys me :P I'm more interested in 1) PaX' full set of
>>markings (-ps for NX, -m for mprotect(), r for randmmap, x for
>>randexec), [...]
>>I guess it works on Exec Shield, but it frightens me that I have to
>>audit every library an executable uses for a PT_GNU_STACK marking to
>>see if it has an executable stack.
> here there are two misconceptions:
> 1) you claim that the manual setting of markings is better than the
> _automatic_ setting of markings in Fedora. Manual setting is a support
> and maintainance nightmare, there can be false positives and false
> negatives as well. Also, manual setting of markings assumes code review
> or 'does this application break' type of feedback - neither is as
> reliable as automatic detection done by the compiler.

PaX has trampoline detection/emulation. I think the toolchain spits out
libraries with -E when there's one. It's not inherited from libraries
to the executable though; but a quick hack to trace down everything from
`ldd` or `readelf` and grab the -E would do the same thing without
giving a fully executable stack.

> 2) you claim that you have to audit everything. You dont have to. It's
> all automatic. _Fedora developers_ (not you) then check the markings and
> reduce the number of executable stacks as much as possible.

And a distribution maintainer could do the same with PaX. Once it's
done it's fairly low maintenance. I know because I've done it myself.
I can determine minimal pax markings on a given binary in about 15
seconds in most cases.

>>[...] ES' NX approximation fails if you relieve protections on a
>>higher mapping-- which confuses me, isn't vsyscall() a high-address
>>executable mapping, which would disable NX protection for the full
>>address space?
> another misconception. Read the patch and you'll see how it's solved.

I've been told it maps vsyscall at a lower address, though didn't
remember until after I hit send. Is this true?

>>Aside from that, I just trust the PaX developer more. He's already
>>got a more developed product; he's a security developer instead of a
>>scheduler developer; and he reads CPU manuals for breakfast.
> this is your choice, and i respect it. Please show the same amount of
> respect for the choice of others as well, without badmouthing anything
> just because their choice is different from yours.

I respect you as a kernel developer as long as you're doing preemption
and schedulers; but I honestly think PaX is the better technology, and I
think it's important that the best security technology be in place. My
concerns are only with real security, and in that respect I feel that if
I didn't stand up and assert my understandings on the matter that people
may hurt themselves. I can't put a slave collar on people and use force
feedback to control their minds, but I don't have to keep quiet either.

It doesn't much matter at this point. If everything goes well, PaX
should show up in a fairly popular distribution soon, so we'll get to
finally see something added that this conversation lacks: a genuine
large-scale demonstration of the deployment of PaX. ES has that
already; but until I can see the excellence and the failings of PaX
deployed with a target on the average user as well, I can't make
assessments about which deploys better in what cases and why.

On a final note, isn't PaX the only technology trying to apply NX
protections to kernel space? Granted, most kernel exploits aren't RCE;
but it's still a basic protection that should be in place. Wouldn't it
be embarassing to say one day that RCE is so rare we don't need to waste
effort on kernel-level W/X separation, then the next day see an RCE
exploit? :P (do it murphy, do it! >:P) This is just a generic
observation; as I said, RCE in kernel is rare enough to not be a major
selling point, but it's still a consideration.

> Ingo
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