Re: [kernel-hardening] Re: [patch v2] move RLIMIT_NPROC check from set_user() to do_execve_common()

Vasiliy Kulikov
2011-07-29T07:08:10Z

On Wed, Jul 27, 2011 at 12:15 +1000, NeilBrown wrote:
> On Tue, 26 Jul 2011 18:48:48 +0400 Vasiliy Kulikov <segoon@openwall.com>
> wrote:
> > On Tue, Jul 26, 2011 at 14:11 +1000, NeilBrown wrote:
> > > I don't really see that failing mmap is any more hackish than failing execve.
> > >
> > > Both are certainly hacks. It is setuid that should fail, but that is
> > > problematic.
> > >
> > > We seem to agree that it is acceptable to delay the failure until the process
> > > actually tries to run some code for the user. I just think that
> > > mapping-a-file-for-exec is a more direct measure of "trying to run some code
> > > for the user" than "execve" is.
> > >
> > > So they are both hacks, but one it more thorough than the other. In the
> > > world of security I would hope that "thorough" would win.
> >
> > Well, I don't mind against something more generic than the check in
> > execve(), however, the usefulness of the check in mmap() is unclear to
> > me. You want to make more programs fail after setuid(), but does mmap
> > stops really many programs? Do you know any program doing mmap/dlopen
> > after setuid() call? What if the program will not do any mmap/dlopen
> > and e.g. start to handle network connections or do some computations?
> > I suppose the latter case is much more often than mmap/dlopen.
>
> I think I didn't make myself clear.
> I don't mean we should intercept the mmap system call.
>
> I mean we could intercept the internal kernel function do_mmap_pgoff.
>
> This is used by the mmap system call but also (and more importantly) by the
> execve system call and the uselib system call.

Here I have 2 doubts:

1) uselib() after setuid() is a rare pattern. If it doesn't catch
relatively many programs, then it is only an additional complexity -
addition the check deeply into mm rather than into obvious do_execve().

2) As it touches mmap(), I'm afraid it could affect mmap-based heap
allocations as well. The problem is that it might be somewhat not
predictable and not repeating event - one time the process did mmap() to
increase the heap, one time it did not because of minor allocation
differences. The former process would be terminated just after mmap()
failed, the latter would not. I don't think such uncertainty is good
(even if it is rare).

So, I'm hesitating to call the check in do_mmap_pgoff() "simply more
generic".

Thanks,

--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments