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

[Kees Cook]

On Thu, Sep 24, 2020 at 04:46:05PM -0400, Paul Moore wrote:
> On Thu, Sep 24, 2020 at 3:52 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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. :)
> 
> What's really the worst that could happen anyways? (/me ducks)  The
> worst case is the filter falls back to the current performance levels
> right?

Worse case for adding complexity to verifier is the bitmaps can be
tricked into a bad state, but I've tried to design this so that it can
only fail toward just running the filter. :)

-- 
Kees Cook