On Fri, Jun 16, 2023 at 3:15 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>
> 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.
Sorry about this, I will fix it when I respin.
>
> > 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().
Ack, will use 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; \
