Re: [PATCH v13 5/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

Paul Moore
2024-07-03T00:07:37Z

On Jun 29, 2024 KP Singh <kpsingh@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@chromium.org>
> Acked-by: Casey Schaufler <casey@schaufler-ca.com>
> Signed-off-by: KP Singh <kpsingh@kernel.org>
> ---
> include/linux/lsm_hooks.h | 30 ++++++++++++++++++++++++++++-
> kernel/bpf/trampoline.c | 40 +++++++++++++++++++++++++++++++++++----
> security/bpf/hooks.c | 2 +-
> security/security.c | 36 ++++++++++++++++++++++++++++++++++-
> 4 files changed, 101 insertions(+), 7 deletions(-)

I didn't look at this one too closely, see my previous comments in
patch 3/5, but I did catch one typo, see below ...

> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index a66ca68485a2..dbe0f40f7f67 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -110,11 +110,14 @@ struct lsm_id {
> * @scalls: The beginning of the array of static calls assigned to this hook.
> * @hook: The callback for the hook.
> * @lsm: The name of the lsm that owns this hook.
> + * @default_state: The state of the LSM hook when initialized. If set to false,
> + * the static key guarding the hook will be set to disabled.
> */
> struct security_hook_list {
> struct lsm_static_call *scalls;
> union security_list_options hook;
> const struct lsm_id *lsmid;
> + bool runtime;
> } __randomize_layout;

The comment header doesn't match the struct fields, "default_state" vs
"runtime".

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