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

[Willy Tarreau]

 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.

Willy