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Subject[PATCH v1 0/6] seccomp: Implement constant action bitmaps
Date
rfc: https://lore.kernel.org/lkml/20200616074934.1600036-1-keescook@chromium.org/
alternative: https://lore.kernel.org/containers/cover.1600661418.git.yifeifz2@illinois.edu/
v1:
- rebase to for-next/seccomp
- finish X86_X32 support for both pinning and bitmaps
- replace TLB magic with Jann's emulator
- add JSET insn

TODO:
- add ALU|AND insn
- significantly more testing

Hi,

This is a refresh of my earlier constant action bitmap series. It looks
like the RFC was missed on the container list, so I've CCed it now. :)
I'd like to work from this series, as it handles the multi-architecture
stuff.

Repeating the commit log from patch 3:

seccomp: Implement constant action bitmaps

One of the most common pain points with seccomp filters has been dealing
with the overhead of processing the filters, especially for "always allow"
or "always reject" cases. While BPF is extremely fast[1], it will always
have overhead associated with it. Additionally, due to seccomp's design,
filters are layered, which means processing time goes up as the number
of filters attached goes up.

In the past, efforts have been focused on making filter execution complete
in a shorter amount of time. For example, filters were rewritten from
using linear if/then/else syscall search to using balanced binary trees,
or moving tests for syscalls common to the process's workload to the
front of the filter. However, there are limits to this, especially when
some processes are dealing with tens of filters[2], or when some
architectures have a less efficient BPF engine[3].

The most common use of seccomp, constructing syscall block/allow-lists,
where syscalls that are always allowed or always rejected (without regard
to any arguments), also tends to produce the most pathological runtime
problems, in that a large number of syscall checks in the filter need
to be performed to come to a determination.

In order to optimize these cases from O(n) to O(1), seccomp can
use bitmaps to immediately determine the desired action. A critical
observation in the prior paragraph bears repeating: the common case for
syscall tests do not check arguments. For any given filter, there is a
constant mapping from the combination of architecture and syscall to the
seccomp action result. (For kernels/architectures without CONFIG_COMPAT,
there is a single architecture.). As such, it is possible to construct
a mapping of arch/syscall to action, which can be updated as new filters
are attached to a process.

In order to build this mapping at filter attach time, each filter is
executed 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, then
there is a constant mapping for that syscall, and bitmaps can be updated
accordingly. If any accesses happen outside of those struct members,
seccomp must not bypass filter execution for that syscall, since program
state will be used to determine filter action result. (This logic comes
in the next patch.)

[1] https://lore.kernel.org/bpf/20200531171915.wsxvdjeetmhpsdv2@ast-mbp.dhcp.thefacebook.com/
[2] https://lore.kernel.org/bpf/20200601101137.GA121847@gardel-login/
[3] https://lore.kernel.org/bpf/717a06e7f35740ccb4c70470ec70fb2f@huawei.com/


Thanks!

-Kees


Kees Cook (6):
seccomp: Introduce SECCOMP_PIN_ARCHITECTURE
x86: Enable seccomp architecture tracking
seccomp: Implement constant action bitmaps
seccomp: Emulate basic filters for constant action results
selftests/seccomp: Compare bitmap vs filter overhead
[DEBUG] seccomp: Report bitmap coverage ranges

arch/x86/include/asm/seccomp.h | 14 +
include/linux/seccomp.h | 27 +
include/uapi/linux/seccomp.h | 1 +
kernel/seccomp.c | 473 +++++++++++++++++-
net/core/filter.c | 3 +-
.../selftests/seccomp/seccomp_benchmark.c | 151 +++++-
tools/testing/selftests/seccomp/seccomp_bpf.c | 33 ++
tools/testing/selftests/seccomp/settings | 2 +-
8 files changed, 674 insertions(+), 30 deletions(-)

--
2.25.1

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