On Fri, Oct 06, 2023 at 10:47:01PM +0200, KP Singh wrote: > 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> > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -- Kees Cook
