[PATCH v13 5/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[Paul Moore]

On Jun 29, 2024 KP Singh <kpsingh at kernel.org> 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_RUNTIME variant
> that allows the LSMs to opt-in to hooks which can be toggled at runtime
> which with security_toogle_hook.
> 
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> Signed-off-by: KP Singh <kpsingh at kernel.org>
> ---
>  include/linux/lsm_hooks.h | 30 ++++++++++++++++++++++++++++-
>  kernel/bpf/trampoline.c   | 40 +++++++++++++++++++++++++++++++++++----
>  security/bpf/hooks.c      |  2 +-
>  security/security.c       | 36 ++++++++++++++++++++++++++++++++++-
>  4 files changed, 101 insertions(+), 7 deletions(-)

I didn't look at this one too closely, see my previous comments in
patch 3/5, but I did catch one typo, see below ...

> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index a66ca68485a2..dbe0f40f7f67 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -110,11 +110,14 @@ struct lsm_id {
>   * @scalls: The beginning of the array of static calls assigned to this hook.
>   * @hook: The callback for the hook.
>   * @lsm: The name of the lsm that owns this hook.
> + * @default_state: The state of the LSM hook when initialized. If set to false,
> + * the static key guarding the hook will be set to disabled.
>   */
>  struct security_hook_list {
>  	struct lsm_static_call	*scalls;
>  	union security_list_options	hook;
>  	const struct lsm_id		*lsmid;
> +	bool				runtime;
>  } __randomize_layout;

The comment header doesn't match the struct fields, "default_state" vs
"runtime".

--
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