Re: [PATCH v5 seccomp 2/5] seccomp/cache: Add "emulator" to check if filter is constant allow

[Jann Horn]

On Sun, Oct 11, 2020 at 5:48 PM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> SECCOMP_CACHE 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.
>
> In order to build this bitmap at filter attach time, each filter is
> emulated for every syscall (under each possible architecture), and
> checked for any accesses of struct seccomp_data that are not the "arch"
> nor "nr" (syscall) members. If only "arch" and "nr" are examined, and
> the program returns allow, then we can be sure that the filter must
> return allow independent from syscall arguments.
>
> Nearly all seccomp filters are built from these cBPF instructions:
>
> 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
> BPF_ALU | BPF_AND  | BPF_K
>
> Each of these instructions 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@xxxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@xxxxxxxxxxxxxx/
>
> Suggested-by: Jann Horn <jannh@xxxxxxxxxx>
> Co-developed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>

Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>