[PATCH v11 5/5] bpf: Only enable BPF LSM hooks when an LSM program is attached
Casey Schaufler
casey at schaufler-ca.com
Fri May 10 17:11:00 UTC 2024
On 5/9/2024 1:14 PM, KP Singh 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_TOGGLEABLE
> variant which allows the LSMs to opt-in to toggleable hooks which can
> be toggled on/off with security_toogle_hook.
>
> Signed-off-by: KP Singh <kpsingh at kernel.org>
I know that there's still some bikeshedding to deal with, but
I don't see that significantly affecting the behavior.
Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> ---
> include/linux/lsm_hooks.h | 30 ++++++++++++++++++++++++++++-
> kernel/bpf/trampoline.c | 40 +++++++++++++++++++++++++++++++++++----
> security/bpf/hooks.c | 2 +-
> security/security.c | 37 +++++++++++++++++++++++++++++++++++-
> 4 files changed, 102 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 5db244308c92..5c0918ed6b80 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 toggleable;
> } __randomize_layout;
>
> /*
> @@ -164,7 +167,19 @@ static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
> #define LSM_HOOK_INIT(NAME, HOOK) \
> { \
> .scalls = static_calls_table.NAME, \
> - .hook = { .NAME = HOOK } \
> + .hook = { .NAME = HOOK }, \
> + .toggleable = false \
> + }
> +
> +/*
> + * Toggleable LSM hooks are enabled at runtime with
> + * security_toggle_hook and are initialized as inactive.
> + */
> +#define LSM_HOOK_INIT_TOGGLEABLE(NAME, HOOK) \
> + { \
> + .scalls = static_calls_table.NAME, \
> + .hook = { .NAME = HOOK }, \
> + .toggleable = true \
> }
>
> extern char *lsm_names;
> @@ -206,4 +221,17 @@ extern struct lsm_info __start_early_lsm_info[], __end_early_lsm_info[];
> extern int lsm_inode_alloc(struct inode *inode);
> extern struct lsm_static_calls_table static_calls_table __ro_after_init;
>
> +#ifdef CONFIG_SECURITY
> +
> +int security_toggle_hook(void *addr, bool value);
> +
> +#else
> +
> +static inline int security_toggle_hook(void *addr, bool value)
> +{
> + return -EINVAL;
> +}
> +
> +#endif /* CONFIG_SECURITY */
> +
> #endif /* ! __LINUX_LSM_HOOKS_H */
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index db7599c59c78..5758c5681023 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -521,6 +521,21 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
> }
> }
>
> +static int 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;
> + }
> + }
> + return security_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;
> @@ -560,11 +575,22 @@ 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]++;
> - err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> - if (err) {
> - hlist_del_init(&link->tramp_hlist);
> - tr->progs_cnt[kind]--;
> +
> + if (link->link.prog->type == BPF_PROG_TYPE_LSM) {
> + err = bpf_trampoline_toggle_lsm(tr, kind);
> + if (err)
> + goto cleanup;
> }
> +
> + err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> + if (err)
> + goto cleanup;
> +
> + return 0;
> +
> +cleanup:
> + hlist_del_init(&link->tramp_hlist);
> + tr->progs_cnt[kind]--;
> return err;
> }
>
> @@ -593,6 +619,12 @@ 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) {
> + err = bpf_trampoline_toggle_lsm(tr, kind);
> + WARN(err, "BUG: unable to toggle BPF LSM hook");
> + }
> +
> return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> }
>
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index 57b9ffd53c98..ba1c3a19fb12 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_TOGGLEABLE(NAME, bpf_lsm_##NAME),
> #include <linux/lsm_hook_defs.h>
> #undef LSM_HOOK
> LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
> diff --git a/security/security.c b/security/security.c
> index 491b807a8a63..a89eb8fe302b 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -407,7 +407,9 @@ static void __init lsm_static_call_init(struct security_hook_list *hl)
> __static_call_update(scall->key, scall->trampoline,
> hl->hook.lsm_func_addr);
> scall->hl = hl;
> - static_branch_enable(scall->active);
> + /* Toggleable hooks are inactive by default */
> + if (!hl->toggleable)
> + static_branch_enable(scall->active);
> return;
> }
> scall++;
> @@ -885,6 +887,39 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
> return rc;
> }
>
> +/**
> + * security_toggle_hook - Toggle the state of the LSM hook.
> + * @hook_addr: The address of the hook to be toggled.
> + * @state: Whether to enable for disable the hook.
> + *
> + * Returns 0 on success, -EINVAL if the address is not found.
> + */
> +int security_toggle_hook(void *hook_addr, bool state)
> +{
> + struct lsm_static_call *scalls = ((void *)&static_calls_table);
> + unsigned long num_entries =
> + (sizeof(static_calls_table) / sizeof(struct lsm_static_call));
> + int i;
> +
> + for (i = 0; i < num_entries; i++) {
> + if (!scalls[i].hl->toggleable)
> + continue;
> +
> + if (!scalls[i].hl)
> + continue;
> +
> + if (scalls[i].hl->hook.lsm_func_addr != hook_addr)
> + continue;
> +
> + if (state)
> + static_branch_enable(scalls[i].active);
> + else
> + static_branch_disable(scalls[i].active);
> + return 0;
> + }
> + return -EINVAL;
> +}
> +
> /*
> * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
> * can be accessed with:
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