On Thu, Apr 11, 2024 at 2:38 AM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Feb 7, 2024 KP Singh <kpsingh@xxxxxxxxxx> wrote:
> >
> > BPF LSM hooks have side-effects (even when a default value is 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:
> > 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>: jmp 0xffffffff818f0e57 <security_file_ioctl+39>
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > Static key enabled for SELinux
> >
> > 0xffffffff818f0e47 <+23>: xchg %ax,%ax
> > ^^^^^^^^^^^^^^
> >
> > Static key disabled for BPF. This gets patched when a BPF LSM program
> > is attached
> >
> > 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 0xffffffff819033c0 <selinux_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>: call 0xffffffff8141e3b0 <bpf_lsm_file_ioctl>
> > 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
> >
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > Acked-by: Song Liu <song@xxxxxxxxxx>
> > Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> > ---
> > include/linux/bpf_lsm.h | 5 +++++
> > include/linux/lsm_hooks.h | 13 ++++++++++++-
> > kernel/bpf/trampoline.c | 24 ++++++++++++++++++++++++
> > security/bpf/hooks.c | 25 ++++++++++++++++++++++++-
> > security/security.c | 3 ++-
> > 5 files changed, 67 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> > index 1de7ece5d36d..5bbc31ac948c 100644
> > --- a/include/linux/bpf_lsm.h
> > +++ b/include/linux/bpf_lsm.h
> > @@ -29,6 +29,7 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> >
> > bool bpf_lsm_is_sleepable_hook(u32 btf_id);
> > bool bpf_lsm_is_trusted(const struct bpf_prog *prog);
> > +void bpf_lsm_toggle_hook(void *addr, bool value);
> >
> > static inline struct bpf_storage_blob *bpf_inode(
> > const struct inode *inode)
> > @@ -78,6 +79,10 @@ static inline void bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog,
> > {
> > }
> >
> > +static inline void bpf_lsm_toggle_hook(void *addr, bool value)
> > +{
> > +}
> > +
> > #endif /* CONFIG_BPF_LSM */
> >
> > #endif /* _LINUX_BPF_LSM_H */
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index ba63d8b54448..e95f0a5cb409 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 default_enabled;
>
> Ugh. We've already got an lsm_static_call::active field, I don't want
> to see another enable/active/etc. flag unless there is absolutely no way
> this works otherwise.
The field default_enabled is used at the time of initialization. The
lsm_static_call::active is a static key which we really cannot use at
initialization time from the various LSMs directly. I don't see a way
out of this one IMHO.
>
> > } __randomize_layout;
> >
> > /*
> > @@ -164,7 +167,15 @@ static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
> > #define LSM_HOOK_INIT(NAME, CALLBACK) \
> > { \
> > .scalls = static_calls_table.NAME, \
> > - .hook = { .NAME = CALLBACK } \
> > + .hook = { .NAME = CALLBACK }, \
> > + .default_enabled = true \
> > + }
> > +
> > +#define LSM_HOOK_INIT_DISABLED(NAME, CALLBACK) \
> > + { \
> > + .scalls = static_calls_table.NAME, \
> > + .hook = { .NAME = CALLBACK }, \
[...]
static_branch_disable(scalls->active);
> > + }
> > + }
> > +}
>
> More ugh. If we're going to solve things this way, let's make it a
> proper LSM interface and not a BPF LSM specific hack; I *really* don't
> want to see individual LSMs managing the lsm_static_call or
> security_hook_list entries.
>
Fair, and that makes the implementation much simpler too. I created a
security_hook_toggle function in security.c which implements this
functionality.
- KP
> > diff --git a/security/security.c b/security/security.c
> > index e05d2157c95a..40d83da87f68 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -406,7 +406,8 @@ static void __init lsm_static_call_init(struct security_hook_list *hl)
> > __static_call_update(scall->key, scall->trampoline,
> > hl->hook.lsm_callback);
> > scall->hl = hl;
> > - static_branch_enable(scall->active);
> > + if (hl->default_enabled)
> > + static_branch_enable(scall->active);
> > return;
> > }
> > scall++;
> > --
> > 2.43.0.594.gd9cf4e227d-goog
>
> --
> paul-moore.com
