Re: [PATCH 4/3] x86/ldt: allow to disable modify_ldt at runtime

[Willy Tarreau <w@1wt.eu>]

On Mon, Jul 27, 2015 at 12:04:54PM -0700, Kees Cook wrote:
> On Sat, Jul 25, 2015 at 6:03 AM, Willy Tarreau <w@1wt.eu> wrote:
> > On Sat, Jul 25, 2015 at 09:50:52AM +0200, Willy Tarreau wrote:
> >> On Fri, Jul 24, 2015 at 11:44:52PM -0700, Andy Lutomirski wrote:
> >> > I'm all for it, but I think it should be hard-disablable in config,
> >> > too, for the -tiny people.
> >>
> >> I totally agree.
> >>
> >> > If we add a runtime disable, let's do a
> >> > separate patch, and you and Kees can fight over how general it should
> >> > be.
> >>
> >> Initially I was thinking about changing it for a 3-state option but
> >> that would prevent X86_16BIT from being hard-disablable, so I'll do
> >> something completely separate.
> >
> > So here comes the proposed patch. It adds a default setting for the
> > sysctl when the option is not hard-disabled (eg: distros not wanting
> > to take risks with legacy apps). It suggests to leave the option off.
> > In case a syscall is blocked, a printk_ratelimited() is called with
> > relevant info (program name, pid, uid) so that the admin can decide
> > whether it's a legitimate call or not. Eg:
> >
> >   Denied a call to modify_ldt() from a.out[1736] (uid: 100). Adjust sysctl if this was not an exploit attempt.
> >
> > I personally think it completes well your series, hence the 4/3 numbering.
> > Feel free to adopt it if you cycle another round and if you're OK with it
> > of course.
> >
> > CCing Kees as well.
> 
> This patch looks reasonable, but I'd prefer a tri-state (enable,
> disable, hard-disable).

That was my first goal initially until I realized that the current two
options make it possible to also get rid of X86_16BIT as Andy did. I
don't see how to do this with the 3-state mode.

> I do something like this for Yama's ptrace
> zero to max_scope range (which "pins" to max_scope if set):
> 
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/security/yama/yama_lsm.c#n361

I agree with this and initially I intended to do something approximately
like this when I realized that for this specific case it didn't match the
pattern. In fact here we have the opportunity to completely remove support
for LDT changes, not just the modify_ldt() syscall. Then it makes sense to
have the two options here.

Regards,
Willy