On Wed, Nov 1, 2023 at 5:55 PM 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>
> 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..b646f6746147 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_state;
minor nit: "default_state" would make more sense if it would be some
enum instead of bool. But given it's true/false, default_enabled makes
more sense.
> } __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_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 91011e0c361a..61433633d235 100644
> --- a/security/bpf/hooks.c
> +++ b/security/bpf/hooks.c
> @@ -9,7 +9,7 @@
>
> 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),
> @@ -39,3 +39,26 @@ DEFINE_LSM(bpf) = {
> .init = bpf_lsm_init,
> .blobs = &bpf_lsm_blob_sizes
> };
> +
> +void bpf_lsm_toggle_hook(void *addr, bool value)
another minor nit: similar to above, s/value/enable/ reads nicer
> +{
> + 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 aa3c87257533..58e93614037f 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_state)
> + static_branch_enable(scall->active);
> return;
> }
> scall++;
> --
> 2.42.0.820.g83a721a137-goog
>
>
