[PATCH v2 seccomp 0/6] seccomp: Add bitmap cache of arg-independent filter results that allow syscalls

YiFei Zhu
2020-09-24T12:44:45Z

From: YiFei Zhu <yifeifz2@illinois.edu>

Alternative: https://lore.kernel.org/lkml/20200923232923.3142503-1-keescook@chromium.org/T/

Major differences from the linked alternative by Kees:
* No x32 special-case handling -- not worth the complexity
* No caching of denylist -- not worth the complexity
* No seccomp arch pinning -- I think this is an independent feature
* The bitmaps are part of the filters rather than the task.
* Architectures supported by default through arch number array,
except for MIPS with its sparse syscall numbers.
* Configurable per-build for future different cache modes.

This series adds a bitmap to cache seccomp filter results if the
result permits a syscall and is indepenent of syscall arguments.
This visibly decreases seccomp overhead for most common seccomp
filters with very little memory footprint.

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.

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.

When it is concluded that an allow must occur for the given
architecture and syscall pair, seccomp will immediately allow
the syscall, bypassing further BPF execution.

Ongoing work is to further support arguments with fast hash table
lookups. We are investigating the performance of doing so [6], and how
to best integrate with the existing seccomp infrastructure.

Some benchmarks are performed with results in patch 5, copied below:
Current BPF sysctl settings:
net.core.bpf_jit_enable = 1
net.core.bpf_jit_harden = 0
Benchmarking 100000000 syscalls...
63.896255358 - 0.008504529 = 63887750829 (63.9s)
getpid native: 638 ns
130.383312423 - 63.897315189 = 66485997234 (66.5s)
getpid RET_ALLOW 1 filter (bitmap): 664 ns
196.789080421 - 130.384414983 = 66404665438 (66.4s)
getpid RET_ALLOW 2 filters (bitmap): 664 ns
268.844643304 - 196.790234168 = 72054409136 (72.1s)
getpid RET_ALLOW 3 filters (full): 720 ns
342.627472515 - 268.845799103 = 73781673412 (73.8s)
getpid RET_ALLOW 4 filters (full): 737 ns
Estimated total seccomp overhead for 1 bitmapped filter: 26 ns
Estimated total seccomp overhead for 2 bitmapped filters: 26 ns
Estimated total seccomp overhead for 3 full filters: 82 ns
Estimated total seccomp overhead for 4 full filters: 99 ns
Estimated seccomp entry overhead: 26 ns
Estimated seccomp per-filter overhead (last 2 diff): 17 ns
Estimated seccomp per-filter overhead (filters / 4): 18 ns
Expectations:
native ≤ 1 bitmap (638 ≤ 664): ✔️
native ≤ 1 filter (638 ≤ 720): ✔️
per-filter (last 2 diff) ≈ per-filter (filters / 4) (17 ≈ 18): ✔️
1 bitmapped ≈ 2 bitmapped (26 ≈ 26): ✔️
entry ≈ 1 bitmapped (26 ≈ 26): ✔️
entry ≈ 2 bitmapped (26 ≈ 26): ✔️
native + entry + (per filter * 4) ≈ 4 filters total (732 ≈ 737): ✔️

RFC -> v1:
* Config made on by default across all arches that could support it.
* Added arch numbers array and emulate filter for each arch number, and
have a per-arch bitmap.
* Massively simplified the emulator so it would only support the common
instructions in Kees's list.
* Fixed inheriting bitmap across filters (filter->prev is always NULL
during prepare).
* Stole the selftest from Kees.
* Added a /proc/pid/seccomp_cache by Jann's suggestion.

v1 -> v2:
* Corrected one outdated function documentation.

Patch 1 moves the SECCOMP Kcomfig option to arch/Kconfig.

Patch 2 adds a syscall_arches array so the emulator can enumerate it.

Patch 3 implements the emulator that finds if a filter must return allow,

Patch 4 implements the test_bit against the bitmaps.

Patch 5 updates the selftest to better show the new semantics.

Patch 6 implements /proc/pid/seccomp_cache.

[1] https://lore.kernel.org/linux-security-module/c22a6c3cefc2412cad00ae14c1371711@huawei.com/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

Kees Cook (1):
selftests/seccomp: Compare bitmap vs filter overhead

YiFei Zhu (5):
seccomp: Move config option SECCOMP to arch/Kconfig
asm/syscall.h: Add syscall_arches[] array
seccomp/cache: Add "emulator" to check if filter is arg-dependent
seccomp/cache: Lookup syscall allowlist for fast path
seccomp/cache: Report cache data through /proc/pid/seccomp_cache

arch/Kconfig | 56 ++++
arch/alpha/include/asm/syscall.h | 4 +
arch/arc/include/asm/syscall.h | 24 +-
arch/arm/Kconfig | 15 +-
arch/arm/include/asm/syscall.h | 4 +
arch/arm64/Kconfig | 13 -
arch/arm64/include/asm/syscall.h | 4 +
arch/c6x/include/asm/syscall.h | 13 +-
arch/csky/Kconfig | 13 -
arch/csky/include/asm/syscall.h | 4 +
arch/h8300/include/asm/syscall.h | 4 +
arch/hexagon/include/asm/syscall.h | 4 +
arch/ia64/include/asm/syscall.h | 4 +
arch/m68k/include/asm/syscall.h | 4 +
arch/microblaze/Kconfig | 18 +-
arch/microblaze/include/asm/syscall.h | 4 +
arch/mips/Kconfig | 17 --
arch/mips/include/asm/syscall.h | 16 ++
arch/nds32/include/asm/syscall.h | 13 +-
arch/nios2/include/asm/syscall.h | 4 +
arch/openrisc/include/asm/syscall.h | 4 +
arch/parisc/Kconfig | 16 --
arch/parisc/include/asm/syscall.h | 7 +
arch/powerpc/Kconfig | 17 --
arch/powerpc/include/asm/syscall.h | 14 +
arch/riscv/Kconfig | 13 -
arch/riscv/include/asm/syscall.h | 14 +-
arch/s390/Kconfig | 17 --
arch/s390/include/asm/syscall.h | 7 +
arch/sh/Kconfig | 16 --
arch/sh/include/asm/syscall_32.h | 17 +-
arch/sparc/Kconfig | 18 +-
arch/sparc/include/asm/syscall.h | 9 +
arch/um/Kconfig | 16 --
arch/x86/Kconfig | 16 --
arch/x86/include/asm/syscall.h | 11 +
arch/x86/um/asm/syscall.h | 14 +-
arch/xtensa/Kconfig | 14 -
arch/xtensa/include/asm/syscall.h | 4 +
fs/proc/base.c | 7 +-
include/linux/seccomp.h | 5 +
kernel/seccomp.c | 257 +++++++++++++++++-
.../selftests/seccomp/seccomp_benchmark.c | 151 ++++++++--
tools/testing/selftests/seccomp/settings | 2 +-
44 files changed, 639 insertions(+), 265 deletions(-)

--
2.28.0