Re: [PATCH v5 seccomp 1/5] seccomp/cache: Lookup syscall allowlist bitmap for fast path

[Jann Horn]

On Sun, Oct 11, 2020 at 5:48 PM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> The overhead of running Seccomp filters has been part of some past
> discussions [1][2][3]. Oftentimes, the filters have a large number
> of instructions that check syscall numbers one by one and jump based
> on that. Some users chain BPF filters which further enlarge the
> overhead. A recent work [6] comprehensively measures the Seccomp
> overhead and shows that the overhead is non-negligible and has a
> non-trivial impact on application performance.
>
> We observed some common filters, such as docker's [4] or
> systemd's [5], will make most decisions based only on the syscall
> numbers, and as past discussions considered, a bitmap where each bit
> represents a syscall makes most sense for these filters.
>
> The fast (common) path for seccomp should be that the filter permits
> the syscall to pass through, and failing seccomp is expected to be
> an exceptional case; it is not expected for userspace to call a
> denylisted syscall over and over.
>
> When it can be concluded that an allow must occur for the given
> architecture and syscall pair (this determination is introduced in
> the next commit), seccomp will immediately allow the syscall,
> bypassing further BPF execution.
>
> Each architecture number has its own bitmap. The architecture
> number in seccomp_data is checked against the defined architecture
> number constant before proceeding to test the bit against the
> bitmap with the syscall number as the index of the bit in the
> bitmap, and if the bit is set, seccomp returns allow. The bitmaps
> are all clear in this patch and will be initialized in the next
> commit.
>
> When only one architecture exists, the check against architecture
> number is skipped, suggested by Kees Cook [7].
>
> [1] https://lore.kernel.org/linux-security-module/c22a6c3cefc2412cad00ae14c1371711@xxxxxxxxxx/T/
> [2] https://lore.kernel.org/lkml/202005181120.971232B7B@keescook/T/
> [3] https://github.com/seccomp/libseccomp/issues/116
> [4] https://github.com/moby/moby/blob/ae0ef82b90356ac613f329a8ef5ee42ca923417d/profiles/seccomp/default.json
> [5] https://github.com/systemd/systemd/blob/6743a1caf4037f03dc51a1277855018e4ab61957/src/shared/seccomp-util.c#L270
> [6] Draco: Architectural and Operating System Support for System Call Security
>     https://tianyin.github.io/pub/draco.pdf, MICRO-53, Oct. 2020
> [7] https://lore.kernel.org/bpf/202010091614.8BB0EB64@keescook/
>
> Co-developed-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>

Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>