[PATCH v12 5/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[Paul Moore]

On May 15, 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       | 35 +++++++++++++++++++++++++++++++++-
>  4 files changed, 100 insertions(+), 7 deletions(-)

...

> diff --git a/security/security.c b/security/security.c
> index 9654ca074aed..2f8bcacf1fb4 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -885,6 +887,37 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
>  	return rc;
>  }
>  
> +/**
> + * security_toggle_hook - Toggle the state of the LSM hook.
> + * @hook_addr: The address of the hook to be toggled.
> + * @state: Whether to enable for disable the hook.
> + *
> + * Returns 0 on success, -EINVAL if the address is not found.
> + */
> +int security_toggle_hook(void *hook_addr, bool state)
> +{
> +	struct lsm_static_call *scalls = ((void *)&static_calls_table);

GCC (v14.1.1 if that matters) is complaining about casting randomized
structs.  Looking quickly at the two structs, lsm_static_call and
lsm_static_calls_table, I suspect the cast is harmless even if the
randstruct case, but I would like to see some sort of fix for this so
I don't get spammed by GCC every time I do a build.  On the other hand,
if this cast really is a problem in the randstruct case we obviously
need to fix that.

Either way, resolve this and make sure you test with GCC/randstruct
enabled.

> +	unsigned long num_entries =
> +		(sizeof(static_calls_table) / sizeof(struct lsm_static_call));
> +	int i;
> +
> +	for (i = 0; i < num_entries; i++) {
> +
> +		if (!scalls[i].hl || !scalls[i].hl->runtime)
> +			continue;
> +
> +		if (scalls[i].hl->hook.lsm_func_addr != hook_addr)
> +			continue;
> +
> +		if (state)
> +			static_branch_enable(scalls[i].active);
> +		else
> +			static_branch_disable(scalls[i].active);
> +		return 0;
> +	}
> +	return -EINVAL;
> +}
> +
>  /*
>   * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
>   * can be accessed with:
> -- 
> 2.45.0.rc1.225.g2a3ae87e7f-goog

--
paul-moore.com