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

[Eric Paris]

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   :)