Re: JIT emulator needs

[Bodo Eggert <7eggert@gmx.de>]

Albert Cahalan <acahalan@gmail.com> wrote:
> On 6/19/07, William Lee Irwin III <wli@holomorphy.com> wrote:
>> On Fri, Jun 08, 2007 at 02:35:22AM -0400, Albert Cahalan wrote:

>>> Right now, Linux isn't all that friendly to JIT emulators.
>>> Here are the problems and suggestions to improve the situation.
>>> There is an SE Linux execmem restriction that enforces W^X.
>>> Assuming you don't wish to just disable SE Linux, there are
>>> two ugly ways around the problem. You can mmap a file twice,
>>> or you can abuse SysV shared memory. The mmap method requires
>>> that you know of a filesystem mounted rw,exec where you can
>>> write a very large temporary file. This arbitrary filesystem,
>>> rather than swap space, will be the backing store. The SysV
>>> shared memory method requires an undocumented flag and is
>>> subject to some annoying size limits. Both methods create
>>> objects that will fail to be deleted if the program dies
>>> before marking the objects for deletion.
>>
>> If the policy forbidding self-modifying code lacks a method of
>> exempting programs such as JIT interpreters (which I doubt) then
>> it's a problem. I'm with Alan on this one.
> 
> It does and it doesn't. There is not a reasonable way for a
> user to mark an app as needing full self-modifying ability.
> It's not like the executable stack, which can be set via the
> ELF note markings on the executable. (ELF note markings are
> ideal because they can not be used via a ret-to-libc attack)
> 
> With admin privs, one can change SE Linux settings. Mark the
> executable, disable the protection system-wide, generate a
> completely new SE Linux policy, or just turn SE Linux off.

According to the documents I found about SELinux, you can also
 - create a this-app-needs-selfmodification type
 - allow users to change the context type of their files to this type
 - configure a domain to allow self-modification
 - configure the domain transition

Brave words from someone who did not yet successfully find the magic in
order to install the refpolicy on debilian (after finding their refpolicy-foo
to be incomplete and their refpolicy-src to not compile).
-- 
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It's one of those "evolutionary things" that allows them to stand
closer to the kitchen sink.
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