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
Acked-by: Song Liu <song@xxxxxxxxxx>
Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
---
security/Kconfig | 11 +++++++++++
security/security.c | 6 ++++--
2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/security/Kconfig b/security/Kconfig
index 52c9af08ad35..317018dcbc67 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 && EXPERT
+ default SECURITY_SELINUX || SECURITY_SMACK || SECURITY_TOMOYO || SECURITY_APPARMOR
+ 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 d1ee72e563cc..b8eac2e8a59d 100644
--- a/security/security.c
+++ b/security/security.c
@@ -825,7 +825,8 @@ 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 (static_branch_maybe(CONFIG_SECURITY_HOOK_LIKELY, \
+ &SECURITY_HOOK_ACTIVE_KEY(HOOK, NUM))) { \
static_call(LSM_STATIC_CALL(HOOK, NUM))(__VA_ARGS__); \
} \
} while (0);
@@ -837,7 +838,8 @@ do { \
#define __CALL_STATIC_INT(NUM, R, HOOK, LABEL, ...) \
do { \
- if (static_branch_unlikely(&SECURITY_HOOK_ACTIVE_KEY(HOOK, NUM))) { \
+ if (static_branch_maybe(CONFIG_SECURITY_HOOK_LIKELY, \
+ &SECURITY_HOOK_ACTIVE_KEY(HOOK, NUM))) { \
R = static_call(LSM_STATIC_CALL(HOOK, NUM))(__VA_ARGS__); \
if (R != 0) \
goto LABEL; \
--
2.42.0.515.g380fc7ccd1-goog
