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

[Jann Horn <jannh@google.com>]

On Thu, Sep 24, 2020 at 1:29 AM Kees Cook <keescook@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?

> Suggested-by: Jann Horn <jannh@google.com>
> Link: https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/
> Signed-off-by: Kees Cook <keescook@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);

[...]
> 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.