Re: [PATCH 2/3] [RFC] seccomp: give BPF x32 bit when restoring x32 filter

From: Paul Moore
Date: Fri Jul 11 2014 - 12:21:26 EST

On Friday, July 11, 2014 12:16:47 PM Eric Paris wrote:
> On Fri, 2014-07-11 at 12:11 -0400, Paul Moore wrote:
> > On Thursday, July 10, 2014 09:06:02 PM H. Peter Anvin wrote:
> > > Incidentally: do seccomp users know that on an x86-64 system you can
> > > recevie system calls from any of the x86 architectures, regardless of
> > > how the program is invoked? (This is unusual, so normally denying those
> > > "alien" calls is the right thing to do.)
> >
> > I obviously can't speak for all seccomp users, but libseccomp handles this
> > by checking the seccomp_data->arch value at the start of the filter and
> > killing (by default) any non-native architectures. If you want, you can
> > change this default behavior or add support for other architectures (e.g.
> > create a filter that allows both x86-64 and x32 but disallows x86, or any
> > combination of the three for that matter).
> Maybe libseccomp does some HORRIFIC contortions under the hood, but the
> interface is crap... Since seccomp_data->arch can't distinguish between
> X32 and X86_64. If I write a seccomp filter which says
> KILL arch != x86_64
> KILL init_module
> ALLOW everything else
> I can still call init_module, I just have to use the X32 variant.
> If libseccomp is translating:
> KILL arch != x86_64 into:
> KILL arch != x86_64
> KILL syscall_nr >= 2000
> That's just showing how dumb the kernel interface is... Good for you
> guys, but the kernel is just being dumb :)

You're not going to hear me ever say that I like how the x32 ABI was done, it
is a real mess from a seccomp filter point of view and we have to do some
nasty stuff in libseccomp to make it all work correctly (see my comments on
the libseccomp-devel list regarding my severe displeasure over x32), but
what's done is done.

I think it's too late to change the x32 seccomp filter ABI.

paul moore
security and virtualization @ redhat

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