[PATCH v3 seccomp 2/5] seccomp/cache: Add "emulator" to check if filter is constant allow

[Kees Cook]

On Thu, Oct 01, 2020 at 12:24:32AM +0200, Jann Horn wrote:
> On Wed, Sep 30, 2020 at 5:20 PM YiFei Zhu <zhuyifei1999 at gmail.com> wrote:
> > SECCOMP_CACHE_NR_ONLY will only operate on syscalls that do not
> > access any syscall arguments or instruction pointer. To facilitate
> > this we need a static analyser to know whether a filter will
> > return allow regardless of syscall arguments for a given
> > architecture number / syscall number pair. This is implemented
> > here with a pseudo-emulator, and stored in a per-filter bitmap.
> >
> > Each common BPF instruction are emulated. Any weirdness or loading
> > from a syscall argument will cause the emulator to bail.
> >
> > The emulation is also halted if it reaches a return. In that case,
> > if it returns an SECCOMP_RET_ALLOW, the syscall is marked as good.
> >
> > Emulator structure and comments are from Kees [1] and Jann [2].
> >
> > Emulation is done at attach time. If a filter depends on more
> > filters, and if the dependee does not guarantee to allow the
> > syscall, then we skip the emulation of this syscall.
> >
> > [1] https://lore.kernel.org/lkml/20200923232923.3142503-5-keescook@chromium.org/
> > [2] https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/
> [...]
> > +static void seccomp_cache_prepare_bitmap(struct seccomp_filter *sfilter,
> > +                                        void *bitmap, const void *bitmap_prev,
> > +                                        size_t bitmap_size, int arch)
> > +{
> > +       struct sock_fprog_kern *fprog = sfilter->prog->orig_prog;
> > +       struct seccomp_data sd;
> > +       int nr;
> > +
> > +       for (nr = 0; nr < bitmap_size; nr++) {
> > +               if (bitmap_prev && !test_bit(nr, bitmap_prev))
> > +                       continue;
> > +
> > +               sd.nr = nr;
> > +               sd.arch = arch;
> > +
> > +               if (seccomp_emu_is_const_allow(fprog, &sd))
> > +                       set_bit(nr, bitmap);
> 
> set_bit() is atomic, but since we only do this at filter setup, before
> the filter becomes globally visible, we don't need atomicity here. So
> this should probably use __set_bit() instead.

Oh yes, excellent point! That will speed this up a bit. When you do
this, please include a comment here describing why its safe to do it
non-atomic. :)

-- 
Kees Cook