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

[Kees Cook]

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?

Paul, Tom, under what cases does libseccomp emit ALU|AND into filters?

> > Suggested-by: Jann Horn <jannh at google.com>
> > Link: https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/
> > Signed-off-by: Kees Cook <keescook at chromium.org>
> > ---
> >  kernel/seccomp.c  | 82 ++++++++++++++++++++++++++++++++++++++++++++---
> >  net/core/filter.c |  3 +-
> >  2 files changed, 79 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > index 111a238bc532..9921f6f39d12 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -610,7 +610,12 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> >  {
> >         struct seccomp_filter *sfilter;
> >         int ret;
> > -       const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE);
> > +       const bool save_orig =
> > +#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(SECCOMP_ARCH)
> > +               true;
> > +#else
> > +               false;
> > +#endif
> 
> You could probably write this as something like:
> 
> const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
> __is_defined(SECCOMP_ARCH);

Ah! Thank you. I went looking for __is_defined() and failed. :)

> 
> [...]
> > diff --git a/net/core/filter.c b/net/core/filter.c
> [...]
> > -static void bpf_release_orig_filter(struct bpf_prog *fp)
> > +void bpf_release_orig_filter(struct bpf_prog *fp)
> >  {
> >         struct sock_fprog_kern *fprog = fp->orig_prog;
> >
> > @@ -1154,6 +1154,7 @@ static void bpf_release_orig_filter(struct bpf_prog *fp)
> >                 kfree(fprog);
> >         }
> >  }
> > +EXPORT_SYMBOL_GPL(bpf_release_orig_filter);
> 
> If this change really belongs into this patch (which I don't think it
> does), please describe why in the commit message.

Yup, more cruft I failed to remove.

-- 
Kees Cook