[PATCH 4/6] seccomp: Emulate basic filters for constant action results

[Kees Cook]

On Thu, Sep 24, 2020 at 11:28:55AM -0400, Paul Moore wrote:
> On Thu, Sep 24, 2020 at 3:46 AM Kees Cook <keescook at chromium.org> wrote:
> > On Thu, Sep 24, 2020 at 01:47:47AM +0200, Jann Horn wrote:
> > > On Thu, Sep 24, 2020 at 1:29 AM Kees Cook <keescook at chromium.org> wrote:
> > > > This emulates absolutely the most basic seccomp filters to figure out
> > > > if they will always give the same results for a given arch/nr combo.
> > > >
> > > > Nearly all seccomp filters are built from the following ops:
> > > >
> > > > BPF_LD  | BPF_W    | BPF_ABS
> > > > BPF_JMP | BPF_JEQ  | BPF_K
> > > > BPF_JMP | BPF_JGE  | BPF_K
> > > > BPF_JMP | BPF_JGT  | BPF_K
> > > > BPF_JMP | BPF_JSET | BPF_K
> > > > BPF_JMP | BPF_JA
> > > > BPF_RET | BPF_K
> > > >
> > > > These are now emulated to check for accesses beyond seccomp_data::arch
> > > > or unknown instructions.
> > > >
> > > > Not yet implemented are:
> > > >
> > > > BPF_ALU | BPF_AND (generated by libseccomp and Chrome)
> > >
> > > BPF_AND is normally only used on syscall arguments, not on the syscall
> > > number or the architecture, right? And when a syscall argument is
> > > loaded, we abort execution anyway. So I think there is no need to
> > > implement those?
> >
> > Is that right? I can't actually tell what libseccomp is doing with
> > ALU|AND. It looks like it's using it for building jump lists?
> 
> There is an ALU|AND op in the jump resolution code, but that is really
> just if libseccomp needs to fixup the accumulator because a code block
> is expecting a masked value (right now that would only be a syscall
> argument, not the syscall number itself).
> 
> > Paul, Tom, under what cases does libseccomp emit ALU|AND into filters?
> 
> Presently the only place where libseccomp uses ALU|AND is when the
> masked equality comparison is used for comparing syscall arguments
> (SCMP_CMP_MASKED_EQ).  I can't honestly say I have any good
> information about how often that is used by libseccomp callers, but if
> I do a quick search on GitHub for "SCMP_CMP_MASKED_EQ" I see 2k worth
> of code hits; take that for whatever it is worth.  Tom may have some
> more/better information.
> 
> Of course no promises on future use :)  As one quick example, I keep
> thinking about adding the instruction pointer to the list of things
> that can be compared as part of a libseccomp rule, and if we do that I
> would expect that we would want to also allow a masked comparison (and
> utilize another ALU|AND bpf op there).  However, I'm not sure how
> useful that would be in practice.

Okay, cool. Thanks for checking on that. It sounds like the arg-less
bitmap optimization can continue to ignore ALU|AND for now. :)

-- 
Kees Cook