Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas

[Kees Cook]

On Sun, May 31, 2020 at 02:03:48PM -0700, Andy Lutomirski wrote:
> On Sun, May 31, 2020 at 11:57 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> >
> > What if there was a special filter type that ran a BPF program on each
> > syscall, and the program was allowed to access user memory to make its
> > decisions, e.g. to look at some list of memory addresses.  But this
> > would explicitly *not* be a security feature -- execve() would remove
> > the filter, and the filter's outcome would be one of redirecting
> > execution or allowing the syscall.  If the "allow" outcome occurs,
> > then regular seccomp filters run.  Obviously the exact semantics here
> > would need some care.
> 
> Let me try to flesh this out a little.
> 
> A task could install a syscall emulation filter (maybe using the
> seccomp() syscall, maybe using something else).  There would be at
> most one such filter per process.  Upon doing a syscall, the kernel
> will first do initial syscall fixups (e.g. SYSENTER/SYSCALL32 magic
> argument translation) and would then invoke the filter.  The filter is
> an eBPF program (sorry Kees) and, as input, it gets access to the

FWIW, I agree: something like this needs to use eBPF -- this isn't
being designed as a security boundary. It's more like eBPF ptrace.

> task's register state and to an indication of which type of syscall
> entry this was.  This will inherently be rather architecture specific
> -- x86 choices could be int80, int80(translated), and syscall64.  (We
> could expose SYSCALL32 separately, I suppose, but SYSENTER is such a
> mess that I'm not sure this would be productive.)  The program can
> access user memory, and it returns one of two results: allow the
> syscall or send SIGSYS.  If the program tries to access user memory
> and faults, the result is SIGSYS.
> 
> (I would love to do this with cBPF, but I'm not sure how to pull this
> off.  Accessing user memory is handy for making the lookup flexible
> enough to detect Windows vs Linux.  It would be *really* nice to
> finally settle the unprivileged eBPF subset discussion so that we can
> figure out how to make eBPF work here.)

And yes, this is the next road-block: finding a way to safely do
unprivileged eBPF.

-- 
Kees Cook