# 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% and I/O and Pipe based system calls benefitting the most. Here are the results of the relevant Unixbench system benchmarks with BPF LSM and SELinux enabled with default policies enabled with and without these patches. Benchmark Delta(%): (+ is better) =============================================================================== Execl Throughput +1.9356 File Write 1024 bufsize 2000 maxblocks +6.5953 Pipe Throughput +9.5499 Pipe-based Context Switching +3.0209 Process Creation +2.3246 Shell Scripts (1 concurrent) +1.4975 System Call Overhead +2.7815 System Benchmarks Index Score (Partial Only): +3.4859 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 More context in the excellent LWN article by Jonathan Corbet [2] [1] https://lore.kernel.org/linux-security-module/20200820164753.3256899-1-jackmanb@xxxxxxxxxxxx/ [2] https://lwn.net/Articles/979683/ # v13 to v14 * Dropped Patch 5 based on the ongoing discussion in https://lore.kernel.org/linux-security-module/20240629084331.3807368-4-kpsingh@xxxxxxxxxx/, BPF LSM will still have default callbacks enabled. * Dropped Patch 4 as recommended by Paul, indirect calls will remain in some LSM hooks for now. https://lore.kernel.org/linux-security-module/20240629084331.3807368-5-kpsingh@xxxxxxxxxx/ * Fixed minor nits in Patch 3 # v12 to v13 * Fixed the GCC note when struct randomization is enabled * Added missing link to BPF LSM side effects discussion in the commit message # v11 to v12 * Casey's feedback on readability of patch 4 * Tetsuo's catch on the bug in Patch 5 * Changed LSM_HOOK_TOGGLEABLE to LSM_HOOK_RUNTIME, the bikeshed is blue now. * Also, as security_toggle_hook relies on iterating over a flat lsm_static_calls_table, I added the __packed attribute. # v10 to v11 * bpf_lsm_toggle_hook to security_toggle_hook with LSM_HOOK_TOGGLEABLE limiting the hooks as the ones that can be toggled. # v9 to v10 * Addressed Paul's comments for Patch 3. I did not remove the acks from this one as changes were minor. * Moved BPF LSM specific hook toggling logic bpf_lsm_toggle_hook to s security_toggle_hook as a generic API. I removed the Ack's from this patch as it's worth another look. * Refactored the non-standard hooks to use static calls. # v8 to v9 Paul, I removed the 5th patch about CONFIG_SECURITY_HOOK_LIKELY and went through all the feedback. I believe it all should be addressed now. But, please let me know if I missed anything. The patches are based on https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/lsm.git (next branch as of 2024-02-07) and resolved a bunch of conflicts. I also added Andrii's series ack to indidividual patches. # v7 to v8 * Addressed Andrii's feedback * Rebased (this seems to have removed the syscall changes). v7 has the required conflict resolution incase the conflicts need to be resolved again. # v6 -> v7 * Rebased with latest LSM id changes merged NOTE: The warning shown by the kernel test bot is spurious, there is no flex array and it seems to come from an older tool chain. https://lore.kernel.org/bpf/202310111711.wLbijitj-lkp@xxxxxxxxx/ # v5 -> v6 * Fix a bug in BPF LSM hook toggle logic. # v4 -> v5 * Rebase to linux-next/master * Fixed the case where MAX_LSM_COUNT comes to zero when just CONFIG_SECURITY is compiled in without any other LSM enabled as reported here: https://lore.kernel.org/bpf/202309271206.d7fb60f9-oliver.sang@xxxxxxxxx # v3 -> v4 * Refactor LSM count macros to use COUNT_ARGS * Change CONFIG_SECURITY_HOOK_LIKELY likely's default value to be based on the LSM enabled and have it depend on CONFIG_EXPERT. There are a lot of subtle options behind CONFIG_EXPERT and this should, hopefully alleviate concerns about yet another knob. * __randomize_layout for struct lsm_static_call and, in addition to the cover letter add performance numbers to 3rd patch and some minor commit message updates. * Rebase to linux-next. # v2 -> v3 * Fixed a build issue on archs which don't have static calls and enable CONFIG_SECURITY. * Updated the LSM_COUNT macros based on Andrii's suggestions. * Changed the security_ prefix to lsm_prefix based on Casey's suggestion. * Inlined static_branch_maybe into lsm_for_each_hook on Kees' feedback. # v1 -> v2 (based on linux-next, next-20230614) * Incorporated suggestions from Kees * Changed the way MAX_LSMs are counted from a binary based generator to a clever header. * Add CONFIG_SECURITY_HOOK_LIKELY to configure the likelihood of LSM hooks. KP Singh (3): kernel: Add helper macros for loop unrolling security: Count the LSMs enabled at compile time security: Replace indirect LSM hook calls with static calls include/linux/args.h | 6 +- include/linux/lsm_count.h | 128 +++++++++++++++++++++++ include/linux/lsm_hooks.h | 53 ++++++++-- include/linux/unroll.h | 36 +++++++ security/security.c | 215 ++++++++++++++++++++++++++------------ 5 files changed, 362 insertions(+), 76 deletions(-) create mode 100644 include/linux/lsm_count.h create mode 100644 include/linux/unroll.h -- 2.45.2.803.g4e1b14247a-goog
