[PATCH v11 5/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[Kees Cook]

On Thu, May 09, 2024 at 10:14:21PM +0200, KP Singh wrote:
> BPF LSM hooks have side-effects (even when a default value's returned)
> as some hooks end up behaving differently due to the very presence of
> the hook.
> 
> The static keys guarding the BPF LSM hooks are disabled by default and
> enabled only when a BPF program is attached implementing the hook
> logic. This avoids the issue of the side-effects and also the minor
> overhead associated with the empty callback.
> 
> security_file_ioctl:
>    0xff...0e30 <+0>:	endbr64
>    0xff...0e34 <+4>:	nopl   0x0(%rax,%rax,1)
>    0xff...0e39 <+9>:	push   %rbp
>    0xff...0e3a <+10>:	push   %r14
>    0xff...0e3c <+12>:	push   %rbx
>    0xff...0e3d <+13>:	mov    %rdx,%rbx
>    0xff...0e40 <+16>:	mov    %esi,%ebp
>    0xff...0e42 <+18>:	mov    %rdi,%r14
>    0xff...0e45 <+21>:	jmp    0xff...0e57 <security_file_ioctl+39>
>    				^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
>    Static key enabled for SELinux
> 
>    0xff...0e47 <+23>:	xchg   %ax,%ax
>    			^^^^^^^^^^^^^^
> 
>    Static key disabled for BPF. This gets patched when a BPF LSM
>    program is attached
> 
>    0xff...0e49 <+25>:	xor    %eax,%eax
>    0xff...0e4b <+27>:	xchg   %ax,%ax
>    0xff...0e4d <+29>:	pop    %rbx
>    0xff...0e4e <+30>:	pop    %r14
>    0xff...0e50 <+32>:	pop    %rbp
>    0xff...0e51 <+33>:	cs jmp 0xff...0000 <__x86_return_thunk>
>    0xff...0e57 <+39>:	endbr64
>    0xff...0e5b <+43>:	mov    %r14,%rdi
>    0xff...0e5e <+46>:	mov    %ebp,%esi
>    0xff...0e60 <+48>:	mov    %rbx,%rdx
>    0xff...0e63 <+51>:	call   0xff...33c0 <selinux_file_ioctl>
>    0xff...0e68 <+56>:	test   %eax,%eax
>    0xff...0e6a <+58>:	jne    0xff...0e4d <security_file_ioctl+29>
>    0xff...0e6c <+60>:	jmp    0xff...0e47 <security_file_ioctl+23>
>    0xff...0e6e <+62>:	endbr64
>    0xff...0e72 <+66>:	mov    %r14,%rdi
>    0xff...0e75 <+69>:	mov    %ebp,%esi
>    0xff...0e77 <+71>:	mov    %rbx,%rdx
>    0xff...0e7a <+74>:	call   0xff...e3b0 <bpf_lsm_file_ioctl>
>    0xff...0e7f <+79>:	test   %eax,%eax
>    0xff...0e81 <+81>:	jne    0xff...0e4d <security_file_ioctl+29>
>    0xff...0e83 <+83>:	jmp    0xff...0e49 <security_file_ioctl+25>
>    0xff...0e85 <+85>:	endbr64
>    0xff...0e89 <+89>:	mov    %r14,%rdi
>    0xff...0e8c <+92>:	mov    %ebp,%esi
>    0xff...0e8e <+94>:	mov    %rbx,%rdx
>    0xff...0e91 <+97>:	pop    %rbx
>    0xff...0e92 <+98>:	pop    %r14
>    0xff...0e94 <+100>:	pop    %rbp
>    0xff...0e95 <+101>:	ret
> 
> This patch enables this by providing a LSM_HOOK_INIT_TOGGLEABLE
> variant which allows the LSMs to opt-in to toggleable hooks which can
> be toggled on/off with security_toogle_hook.
> 
> Signed-off-by: KP Singh <kpsingh at kernel.org>

With the issue Tetsuo noted fixed:

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

-- 
Kees Cook