On Mon, Oct 9, 2023 at 12:11 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Fri, Oct 06, 2023 at 10:47:00PM +0200, KP Singh 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>
> > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
>
> small nit, but looks good
>
> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>
> jirka
>
>
> > ---
> > 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);
>
> nit, this could be static, unless there are future plans ;-)
Actually, this is called from trampoline.c and cannot be static.
- KP
>
> >
> > 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 c77a1859214d..57ffe4eb6d30 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -97,11 +97,14 @@ struct lsm_static_calls_table {
> > * @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 char *lsm;
> > + bool default_state;
> > } __randomize_layout;
> >
> > /*
> > @@ -151,7 +154,15 @@ 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_state = true \
> > + }
> > +
> > +#define LSM_HOOK_INIT_DISABLED(NAME, CALLBACK) \
> > + { \
> > + .scalls = static_calls_table.NAME, \
> > + .hook = { .NAME = CALLBACK }, \
> > + .default_state = false \
> > }
> >
> > extern char *lsm_names;
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index e97aeda3a86b..44788e2eaa1b 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -13,6 +13,7 @@
> > #include <linux/bpf_verifier.h>
> > #include <linux/bpf_lsm.h>
> > #include <linux/delay.h>
> > +#include <linux/bpf_lsm.h>
> >
> > /* dummy _ops. The verifier will operate on target program's ops. */
> > const struct bpf_verifier_ops bpf_extension_verifier_ops = {
> > @@ -510,6 +511,21 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
> > }
> > }
> >
> > +static void bpf_trampoline_toggle_lsm(struct bpf_trampoline *tr,
> > + enum bpf_tramp_prog_type kind)
> > +{
> > + struct bpf_tramp_link *link;
> > + bool found = false;
> > +
> > + hlist_for_each_entry(link, &tr->progs_hlist[kind], tramp_hlist) {
> > + if (link->link.prog->type == BPF_PROG_TYPE_LSM) {
> > + found = true;
> > + break;
> > + }
> > + }
> > + bpf_lsm_toggle_hook(tr->func.addr, found);
> > +}
> > +
> > static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr)
> > {
> > enum bpf_tramp_prog_type kind;
> > @@ -549,6 +565,10 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
> >
> > hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
> > tr->progs_cnt[kind]++;
> > +
> > + if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > + bpf_trampoline_toggle_lsm(tr, kind);
> > +
> > err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> > if (err) {
> > hlist_del_init(&link->tramp_hlist);
> > @@ -582,6 +602,10 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
> > }
> > hlist_del_init(&link->tramp_hlist);
> > tr->progs_cnt[kind]--;
> > +
> > + if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > + bpf_trampoline_toggle_lsm(tr, kind);
> > +
> > return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> > }
> >
> > diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> > index cfaf1d0e6a5f..47e1a4777ec9 100644
> > --- a/security/bpf/hooks.c
> > +++ b/security/bpf/hooks.c
> > @@ -8,7 +8,7 @@
> >
> > static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
> > #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
> > - LSM_HOOK_INIT(NAME, bpf_lsm_##NAME),
> > + LSM_HOOK_INIT_DISABLED(NAME, bpf_lsm_##NAME),
> > #include <linux/lsm_hook_defs.h>
> > #undef LSM_HOOK
> > LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
> > @@ -32,3 +32,26 @@ DEFINE_LSM(bpf) = {
> > .init = bpf_lsm_init,
> > .blobs = &bpf_lsm_blob_sizes
> > };
> > +
> > +void bpf_lsm_toggle_hook(void *addr, bool value)
> > +{
> > + struct lsm_static_call *scalls;
> > + struct security_hook_list *h;
> > + int i, j;
> > +
> > + for (i = 0; i < ARRAY_SIZE(bpf_lsm_hooks); i++) {
> > + h = &bpf_lsm_hooks[i];
> > + if (h->hook.lsm_callback != addr)
> > + continue;
> > +
> > + for (j = 0; j < MAX_LSM_COUNT; j++) {
> > + scalls = &h->scalls[j];
> > + if (scalls->hl != &bpf_lsm_hooks[i])
> > + continue;
> > + if (value)
> > + static_branch_enable(scalls->active);
> > + else
> > + static_branch_disable(scalls->active);
> > + }
> > + }
> > +}
> > diff --git a/security/security.c b/security/security.c
> > index ce4c0a9107ea..f45e875b6d93 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -382,7 +382,8 @@ 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_state)
> > + static_branch_enable(scall->active);
> > return;
> > }
> > scall++;
> > --
> > 2.42.0.609.gbb76f46606-goog
> >
> >
