Re: [PATCH v6 5/5] security: Add CONFIG_SECURITY_HOOK_LIKELY

[Kees Cook <keescook@chromium.org>]

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@kernel.org>
> Signed-off-by: KP Singh <kpsingh@kernel.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook