Re: [PATCH v2 5/5] security: Add CONFIG_SECURITY_HOOK_LIKELY

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On 6/15/2023 5:04 PM, 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
>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
>  security/Kconfig    | 11 +++++++++++
>  security/security.c | 13 ++++++++-----
>  2 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/security/Kconfig b/security/Kconfig
> index 52c9af08ad35..bd2a0dff991a 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -32,6 +32,17 @@ config SECURITY
>  
>  	  If you are unsure how to answer this question, answer N.
>  
> +config SECURITY_HOOK_LIKELY
> +	bool "LSM hooks are likely to be initialized"
> +	depends on SECURITY
> +	default y
> +	help
> +	  This controls the behaviour of the static keys that guard LSM hooks.
> +	  If LSM hooks are likely to be initialized by LSMs, then one gets
> +	  better performance by enabling this option. However, if the system is
> +	  using an LSM where hooks are much likely to be disabled, one gets
> +	  better performance by disabling this config.
> +
>  config SECURITYFS
>  	bool "Enable the securityfs filesystem"
>  	help
> diff --git a/security/security.c b/security/security.c
> index 4aec25949212..da80a8918e7d 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -99,9 +99,9 @@ static __initdata struct lsm_info *exclusive;
>   * Define static calls and static keys for each LSM hook.
>   */
>  
> -#define DEFINE_LSM_STATIC_CALL(NUM, NAME, RET, ...)			\
> -	DEFINE_STATIC_CALL_NULL(LSM_STATIC_CALL(NAME, NUM),		\
> -				*((RET(*)(__VA_ARGS__))NULL));		\
> +#define DEFINE_LSM_STATIC_CALL(NUM, NAME, RET, ...)               \
> +	DEFINE_STATIC_CALL_NULL(LSM_STATIC_CALL(NAME, NUM),       \
> +				*((RET(*)(__VA_ARGS__))NULL));    \

This is just a cosmetic change, right? Please fix it in the original
patch when you respin, not here. I spent way to long trying to figure out
why you had to make a change.

>  	DEFINE_STATIC_KEY_FALSE(SECURITY_HOOK_ACTIVE_KEY(NAME, NUM));
>  
>  #define LSM_HOOK(RET, DEFAULT, NAME, ...)				\
> @@ -110,6 +110,9 @@ static __initdata struct lsm_info *exclusive;
>  #undef LSM_HOOK
>  #undef DEFINE_LSM_STATIC_CALL
>  
> +#define security_hook_active(n, h) \
> +	static_branch_maybe(CONFIG_SECURITY_HOOK_LIKELY, &SECURITY_HOOK_ACTIVE_KEY(h, n))
> +

Please don't use the security_ prefix here. It's a local macro, use hook_active()
or, if you must, lsm_hook_active().

>  /*
>   * Initialise a table of static calls for each LSM hook.
>   * DEFINE_STATIC_CALL_NULL invocation above generates a key (STATIC_CALL_KEY)
> @@ -816,7 +819,7 @@ static int lsm_superblock_alloc(struct super_block *sb)
>   */
>  #define __CALL_STATIC_VOID(NUM, HOOK, ...)				     \
>  do {									     \
> -	if (static_branch_unlikely(&SECURITY_HOOK_ACTIVE_KEY(HOOK, NUM))) {    \
> +	if (security_hook_active(NUM, HOOK)) {    			     \
>  		static_call(LSM_STATIC_CALL(HOOK, NUM))(__VA_ARGS__);	     \
>  	}								     \
>  } while (0);
> @@ -828,7 +831,7 @@ do {									     \
>  
>  #define __CALL_STATIC_INT(NUM, R, HOOK, LABEL, ...)			     \
>  do {									     \
> -	if (static_branch_unlikely(&SECURITY_HOOK_ACTIVE_KEY(HOOK, NUM))) {  \
> +	if (security_hook_active(NUM, HOOK)) {    \
>  		R = static_call(LSM_STATIC_CALL(HOOK, NUM))(__VA_ARGS__);    \
>  		if (R != 0)						     \
>  			goto LABEL;					     \




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