This config influences the nature of the static key that guards the static call for LSM hooks. When enabled, it indicates that an LSM static call slot is more likely to be initialized. When disabled, it optimizes for the case when static call slot is more likely to be not initialized. When a major LSM like (SELinux, AppArmor, Smack etc) is active on a system the system would benefit from enabling the config. However there are other cases which would benefit from the config being disabled (e.g. a system with a BPF LSM with no hooks enabled by default, or an LSM like loadpin / yama). Ultimately, there is no one-size fits all solution. with CONFIG_SECURITY_HOOK_LIKELY enabled, the inactive / uninitialized case is penalized with a direct jmp (still better than an indirect jmp): function security_file_ioctl: 0xffffffff818f0c80 <+0>: endbr64 0xffffffff818f0c84 <+4>: nopl 0x0(%rax,%rax,1) 0xffffffff818f0c89 <+9>: push %rbp 0xffffffff818f0c8a <+10>: push %r14 0xffffffff818f0c8c <+12>: push %rbx 0xffffffff818f0c8d <+13>: mov %rdx,%rbx 0xffffffff818f0c90 <+16>: mov %esi,%ebp 0xffffffff818f0c92 <+18>: mov %rdi,%r14 0xffffffff818f0c95 <+21>: jmp 0xffffffff818f0ca8 <security_file_ioctl+40> jump to skip the inactive BPF LSM hook. 0xffffffff818f0c97 <+23>: mov %r14,%rdi 0xffffffff818f0c9a <+26>: mov %ebp,%esi 0xffffffff818f0c9c <+28>: mov %rbx,%rdx 0xffffffff818f0c9f <+31>: call 0xffffffff8141e3b0 <bpf_lsm_file_ioctl> 0xffffffff818f0ca4 <+36>: test %eax,%eax 0xffffffff818f0ca6 <+38>: jne 0xffffffff818f0cbf <security_file_ioctl+63> 0xffffffff818f0ca8 <+40>: endbr64 0xffffffff818f0cac <+44>: jmp 0xffffffff818f0ccd <security_file_ioctl+77> jump to skip the empty slot. 0xffffffff818f0cae <+46>: mov %r14,%rdi 0xffffffff818f0cb1 <+49>: mov %ebp,%esi 0xffffffff818f0cb3 <+51>: mov %rbx,%rdx 0xffffffff818f0cb6 <+54>: nopl 0x0(%rax,%rax,1) ^^^^^^^^^^^^^^^^^^^^^^^ Empty slot 0xffffffff818f0cbb <+59>: test %eax,%eax 0xffffffff818f0cbd <+61>: je 0xffffffff818f0ccd <security_file_ioctl+77> 0xffffffff818f0cbf <+63>: endbr64 0xffffffff818f0cc3 <+67>: pop %rbx 0xffffffff818f0cc4 <+68>: pop %r14 0xffffffff818f0cc6 <+70>: pop %rbp 0xffffffff818f0cc7 <+71>: cs jmp 0xffffffff82c00000 <__x86_return_thunk> 0xffffffff818f0ccd <+77>: endbr64 0xffffffff818f0cd1 <+81>: xor %eax,%eax 0xffffffff818f0cd3 <+83>: jmp 0xffffffff818f0cbf <security_file_ioctl+63> 0xffffffff818f0cd5 <+85>: mov %r14,%rdi 0xffffffff818f0cd8 <+88>: mov %ebp,%esi 0xffffffff818f0cda <+90>: mov %rbx,%rdx 0xffffffff818f0cdd <+93>: pop %rbx 0xffffffff818f0cde <+94>: pop %r14 0xffffffff818f0ce0 <+96>: pop %rbp 0xffffffff818f0ce1 <+97>: ret When the config is disabled, the case optimizes the scenario above. security_file_ioctl: 0xffffffff818f0e30 <+0>: endbr64 0xffffffff818f0e34 <+4>: nopl 0x0(%rax,%rax,1) 0xffffffff818f0e39 <+9>: push %rbp 0xffffffff818f0e3a <+10>: push %r14 0xffffffff818f0e3c <+12>: push %rbx 0xffffffff818f0e3d <+13>: mov %rdx,%rbx 0xffffffff818f0e40 <+16>: mov %esi,%ebp 0xffffffff818f0e42 <+18>: mov %rdi,%r14 0xffffffff818f0e45 <+21>: xchg %ax,%ax 0xffffffff818f0e47 <+23>: xchg %ax,%ax The static keys in their disabled state do not create jumps leading to faster code. 0xffffffff818f0e49 <+25>: xor %eax,%eax 0xffffffff818f0e4b <+27>: xchg %ax,%ax 0xffffffff818f0e4d <+29>: pop %rbx 0xffffffff818f0e4e <+30>: pop %r14 0xffffffff818f0e50 <+32>: pop %rbp 0xffffffff818f0e51 <+33>: cs jmp 0xffffffff82c00000 <__x86_return_thunk> 0xffffffff818f0e57 <+39>: endbr64 0xffffffff818f0e5b <+43>: mov %r14,%rdi 0xffffffff818f0e5e <+46>: mov %ebp,%esi 0xffffffff818f0e60 <+48>: mov %rbx,%rdx 0xffffffff818f0e63 <+51>: call 0xffffffff8141e3b0 <bpf_lsm_file_ioctl> 0xffffffff818f0e68 <+56>: test %eax,%eax 0xffffffff818f0e6a <+58>: jne 0xffffffff818f0e4d <security_file_ioctl+29> 0xffffffff818f0e6c <+60>: jmp 0xffffffff818f0e47 <security_file_ioctl+23> 0xffffffff818f0e6e <+62>: endbr64 0xffffffff818f0e72 <+66>: mov %r14,%rdi 0xffffffff818f0e75 <+69>: mov %ebp,%esi 0xffffffff818f0e77 <+71>: mov %rbx,%rdx 0xffffffff818f0e7a <+74>: nopl 0x0(%rax,%rax,1) 0xffffffff818f0e7f <+79>: test %eax,%eax 0xffffffff818f0e81 <+81>: jne 0xffffffff818f0e4d <security_file_ioctl+29> 0xffffffff818f0e83 <+83>: jmp 0xffffffff818f0e49 <security_file_ioctl+25> 0xffffffff818f0e85 <+85>: endbr64 0xffffffff818f0e89 <+89>: mov %r14,%rdi 0xffffffff818f0e8c <+92>: mov %ebp,%esi 0xffffffff818f0e8e <+94>: mov %rbx,%rdx 0xffffffff818f0e91 <+97>: pop %rbx 0xffffffff818f0e92 <+98>: pop %r14 0xffffffff818f0e94 <+100>: pop %rbp 0xffffffff818f0e95 <+101>: ret Acked-by: Song Liu <song@xxxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx> --- security/Kconfig | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/security/Kconfig b/security/Kconfig index 52c9af08ad35..317018dcbc67 100644 --- a/security/Kconfig +++ b/security/Kconfig @@ -32,6 +32,17 @@ config SECURITY If you are unsure how to answer this question, answer N. +config SECURITY_HOOK_LIKELY + bool "LSM hooks are likely to be initialized" + depends on SECURITY && EXPERT + default SECURITY_SELINUX || SECURITY_SMACK || SECURITY_TOMOYO || SECURITY_APPARMOR + help + This controls the behaviour of the static keys that guard LSM hooks. + If LSM hooks are likely to be initialized by LSMs, then one gets + better performance by enabling this option. However, if the system is + using an LSM where hooks are much likely to be disabled, one gets + better performance by disabling this config. + config SECURITYFS bool "Enable the securityfs filesystem" help -- 2.42.0.869.gea05f2083d-goog
