LSM hooks are currently invoked from a linked list as indirect calls
which are invoked using retpolines as a mitigation for speculative
attacks (Branch History / Target injection) and add extra overhead which
is especially bad in kernel hot paths:

security_file_ioctl:
   0xff...0320 <+0>:	endbr64
   0xff...0324 <+4>:	push   %rbp
   0xff...0325 <+5>:	push   %r15
   0xff...0327 <+7>:	push   %r14
   0xff...0329 <+9>:	push   %rbx
   0xff...032a <+10>:	mov    %rdx,%rbx
   0xff...032d <+13>:	mov    %esi,%ebp
   0xff...032f <+15>:	mov    %rdi,%r14
   0xff...0332 <+18>:	mov    $0xff...7030,%r15
   0xff...0339 <+25>:	mov    (%r15),%r15
   0xff...033c <+28>:	test   %r15,%r15
   0xff...033f <+31>:	je     0xff...0358 <security_file_ioctl+56>
   0xff...0341 <+33>:	mov    0x18(%r15),%r11
   0xff...0345 <+37>:	mov    %r14,%rdi
   0xff...0348 <+40>:	mov    %ebp,%esi
   0xff...034a <+42>:	mov    %rbx,%rdx

   0xff...034d <+45>:	call   0xff...2e0 <__x86_indirect_thunk_array+352>
   			       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Indirect calls that use retpolines leading to overhead, not just due
    to extra instruction but also branch misses.

   0xff...0352 <+50>:	test   %eax,%eax
   0xff...0354 <+52>:	je     0xff...0339 <security_file_ioctl+25>
   0xff...0356 <+54>:	jmp    0xff...035a <security_file_ioctl+58>
   0xff...0358 <+56>:	xor    %eax,%eax
   0xff...035a <+58>:	pop    %rbx
   0xff...035b <+59>:	pop    %r14
   0xff...035d <+61>:	pop    %r15
   0xff...035f <+63>:	pop    %rbp
   0xff...0360 <+64>:	jmp    0xff...47c4 <__x86_return_thunk>

The indirect calls are not really needed as one knows the addresses of
enabled LSM callbacks at boot time and only the order can possibly
change at boot time with the lsm= kernel command line parameter.

An array of static calls is defined per LSM hook and the static calls
are updated at boot time once the order has been determined.

With the hook now exposed as a static call, one can see that the
retpolines are no longer there and the LSM callbacks are invoked
directly:

security_file_ioctl:
   0xff...0ca0 <+0>:	endbr64
   0xff...0ca4 <+4>:	nopl   0x0(%rax,%rax,1)
   0xff...0ca9 <+9>:	push   %rbp
   0xff...0caa <+10>:	push   %r14
   0xff...0cac <+12>:	push   %rbx
   0xff...0cad <+13>:	mov    %rdx,%rbx
   0xff...0cb0 <+16>:	mov    %esi,%ebp
   0xff...0cb2 <+18>:	mov    %rdi,%r14
   0xff...0cb5 <+21>:	jmp    0xff...0cc7 <security_file_ioctl+39>
  			       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   Static key enabled for SELinux

   0xffffffff818f0cb7 <+23>:	jmp    0xff...0cde <security_file_ioctl+62>
   				^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   Static key enabled for BPF LSM. This is something that is changed to
   default to false to avoid the existing side effect issues of BPF LSM
   [1] in a subsequent patch.

   0xff...0cb9 <+25>:	xor    %eax,%eax
   0xff...0cbb <+27>:	xchg   %ax,%ax
   0xff...0cbd <+29>:	pop    %rbx
   0xff...0cbe <+30>:	pop    %r14
   0xff...0cc0 <+32>:	pop    %rbp
   0xff...0cc1 <+33>:	cs jmp 0xff...0000 <__x86_return_thunk>
   0xff...0cc7 <+39>:	endbr64
   0xff...0ccb <+43>:	mov    %r14,%rdi
   0xff...0cce <+46>:	mov    %ebp,%esi
   0xff...0cd0 <+48>:	mov    %rbx,%rdx
   0xff...0cd3 <+51>:	call   0xff...3230 <selinux_file_ioctl>
   			       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   Direct call to SELinux.

   0xff...0cd8 <+56>:	test   %eax,%eax
   0xff...0cda <+58>:	jne    0xff...0cbd <security_file_ioctl+29>
   0xff...0cdc <+60>:	jmp    0xff...0cb7 <security_file_ioctl+23>
   0xff...0cde <+62>:	endbr64
   0xff...0ce2 <+66>:	mov    %r14,%rdi
   0xff...0ce5 <+69>:	mov    %ebp,%esi
   0xff...0ce7 <+71>:	mov    %rbx,%rdx
   0xff...0cea <+74>:	call   0xff...e220 <bpf_lsm_file_ioctl>
   			       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   Direct call to BPF LSM.

   0xff...0cef <+79>:	test   %eax,%eax
   0xff...0cf1 <+81>:	jne    0xff...0cbd <security_file_ioctl+29>
   0xff...0cf3 <+83>:	jmp    0xff...0cb9 <security_file_ioctl+25>
   0xff...0cf5 <+85>:	endbr64
   0xff...0cf9 <+89>:	mov    %r14,%rdi
   0xff...0cfc <+92>:	mov    %ebp,%esi
   0xff...0cfe <+94>:	mov    %rbx,%rdx
   0xff...0d01 <+97>:	pop    %rbx
   0xff...0d02 <+98>:	pop    %r14
   0xff...0d04 <+100>:	pop    %rbp
   0xff...0d05 <+101>:	ret
   0xff...0d06 <+102>:	int3
   0xff...0d07 <+103>:	int3
   0xff...0d08 <+104>:	int3
   0xff...0d09 <+105>:	int3

While this patch uses static_branch_unlikely indicating that an LSM hook
is likely to be not present. In most cases this is still a better choice
as even when an LSM with one hook is added, empty slots are created for
all LSM hooks (especially when many LSMs that do not initialize most
hooks are present on the system).

There are some hooks that don't use the call_int_hook or
call_void_hook. These hooks are updated to use a new macro called
lsm_for_each_hook where the lsm_callback is directly invoked as an
indirect call.

Below are results of the relevant Unixbench system benchmarks with BPF LSM
and SELinux enabled with default policies enabled with and without these
patches.

Benchmark                                          Delta(%): (+ is better)
==========================================================================
Execl Throughput                                             +1.9356
File Write 1024 bufsize 2000 maxblocks                       +6.5953
Pipe Throughput                                              +9.5499
Pipe-based Context Switching                                 +3.0209
Process Creation                                             +2.3246
Shell Scripts (1 concurrent)                                 +1.4975
System Call Overhead                                         +2.7815
System Benchmarks Index Score (Partial Only):                +3.4859

In the best case, some syscalls like eventfd_create benefitted to about
~10%.
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>