# Background LSM hooks (callbacks) are currently invoked as indirect function calls. These callbacks are registered into a linked list at boot time as the order of the LSMs can be configured on the kernel command line with the "lsm=" command line parameter. Indirect function calls have a high overhead due to retpoline mitigation for various speculative execution attacks. Retpolines remain relevant even with newer generation CPUs as recently discovered speculative attacks, like Spectre BHB need Retpolines to mitigate against branch history injection and still need to be used in combination with newer mitigation features like eIBRS. This overhead is especially significant for the "bpf" LSM which allows the user to implement LSM functionality with eBPF program. In order to facilitate this the "bpf" LSM provides a default callback for all LSM hooks. When enabled, the "bpf" LSM incurs an unnecessary / avoidable indirect call. This is especially bad in OS hot paths (e.g. in the networking stack). This overhead prevents the adoption of bpf LSM on performance critical systems, and also, in general, slows down all LSMs. Since we know the address of the enabled LSM callbacks at compile time and only the order is determined at boot time, the LSM framework can allocate static calls for each of the possible LSM callbacks and these calls can be updated once the order is determined at boot. This series is a respin of the RFC proposed by Paul Renauld (renauld@xxxxxxxxxx) and Brendan Jackman (jackmanb@xxxxxxxxxx) [1] # Performance improvement With this patch-set some syscalls with lots of LSM hooks in their path benefitted at an average of ~3%. Here are the results of the relevant Unixbench system benchmarks with BPF LSM and a major LSM (in this case apparmor) enabled with and without the series. Benchmark Delta(%): (+ is better) =============================================================================== Execl Throughput +2.9015 File Write 1024 bufsize 2000 maxblocks +5.4196 Pipe Throughput +7.7434 Pipe-based Context Switching +3.5118 Process Creation +0.3552 Shell Scripts (1 concurrent) +1.7106 System Call Overhead +3.0067 System Benchmarks Index Score (Partial Only): +3.1809 In the best case, some syscalls like eventfd_create benefitted to about ~10%. The full analysis can be viewed at https://kpsingh.ch/lsm-perf [1] https://lore.kernel.org/linux-security-module/20200820164753.3256899-1-jackmanb@xxxxxxxxxxxx/ KP Singh (4): kernel: Add helper macros for loop unrolling security: Generate a header with the count of enabled LSMs security: Replace indirect LSM hook calls with static calls bpf: Only enable BPF LSM hooks when an LSM program is attached include/linux/bpf.h | 1 + include/linux/bpf_lsm.h | 1 + include/linux/lsm_hooks.h | 94 +++++++++++-- include/linux/unroll.h | 35 +++++ kernel/bpf/trampoline.c | 29 ++++- scripts/Makefile | 1 + scripts/security/.gitignore | 1 + scripts/security/Makefile | 4 + scripts/security/gen_lsm_count.c | 57 ++++++++ security/Makefile | 11 ++ security/bpf/hooks.c | 26 +++- security/security.c | 217 ++++++++++++++++++++----------- 12 files changed, 386 insertions(+), 91 deletions(-) create mode 100644 include/linux/unroll.h create mode 100644 scripts/security/.gitignore create mode 100644 scripts/security/Makefile create mode 100644 scripts/security/gen_lsm_count.c -- 2.39.0.246.g2a6d74b583-goog
