Re: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional

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

On Thu, Jul 23, 2015 at 05:09:21PM -0700, Kees Cook wrote:
> > All this to say that probably only a handful of tricky syscalls would
> > need an on/off switch but clearly not all of them at all, so I'd rather
> > add a few entries just for the relevant ones, mainly to fix compatibility
> > issues and nothing more. Eg: what's the point of disabling exit(), wait(),
> > kill(), fork() or getpid()... It would only increase the difficulty to
> > sort out bug reports.
> >
> > Just my opinion,
> 
> Well, I would really like to have something like this around so that I
> can trivially globally disable syscalls when they have security risks.

I understand, but while maybe it could make sense to have the option on
any linux-specific syscall, having it on the standard, portable ones
will be useless as disabling them will break most applications.

> My hack[1] to disable kexec_load, for example, was terrible while I
> waited for a kernel that supported the disable_kexec_load sysctl.

This typically is one linux-specific syscall which no regular application
would rely on and which can come with side effects. I think there are not
*that* many, none of them is performance-critical, and they'd rather be
dealt with one at a time.

> [1] https://outflux.net/blog/archives/2013/12/10/live-patching-the-kernel/

Thanks, that (and the linked articles) was an interesting read.

Willy