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

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.