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

[Paul Moore]

On Friday, July 11, 2014 12:23:33 PM Eric Paris wrote:
> On Fri, 2014-07-11 at 12:21 -0400, Paul Moore wrote:
> > 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.
> 
> So we have a security interface that is damn near impossible to get
> right.  Perfect.

What?  Having to do two comparisons instead of one is "damn near impossible"?  
I think that might be a bit of an overreaction don't you think?

> I think this explains exactly why I support this idea.  Make X32 look
> like everyone else ...

You do realize that this patch set makes x32 the odd man out by having 
syscall_get_nr() return a different syscall number than what was used to make 
the syscall?  I don't understand how that makes "x32 look like everyone else".

> ... and put these custom horrific hacks in seccomp if we are unwilling to
> 'do it right'

If you want to add the new x32 audit arch #define, go for it, like I said that 
was something that I feel should have been in there from the beginning.  As 
far as I'm concerned you can even put a hack in kernel/seccomp.c to rewrite 
the arch token value if it makes your life easier.

> Honestly, how many people are using seccomp on X32 and would be horribly
> pissed if we just fixed it?

Okay, please stop suggesting we break the x32 kernel/user interface to 
workaround a flaw in audit.  I get that it sucks for audit, I really do, but 
this is audit's problem.

-- 
paul moore
security and virtualization @ redhat

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