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

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

On Fri, Jul 24, 2015 at 09:24:51AM +0200, Willy Tarreau wrote:
> 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.

Looking at syscall_64.tbl, I'm seeing that the first ~133 syscalls
have no reason for being disabled if we don't want to break portable
applications, with the exception of ptrace I guess. Past this, things
like uselib, personality, sysfs, prctl etc... could be disabled. There
are still some exceptions in this area but I don't see them as critical
if someone would accidently disable them (eg: getpriority, mlock, ...).
Others like chroot, setrlimit, adjtimex, settimeofday, mount, umount,
time need to be kept. And a few ones like sync or sethostname would be
nice to have optional in order to lock down a system at boot. Many of
the other ones are ns-specific versions of the first ones (*at) and
would rather not being made optional either. I think maybe we can find
between 10 and 30 that would make sense to optionally disable.

Willy