On Wed, May 08, 2024 at 09:00:42AM +0200, KP Singh wrote:
>
>
> > On 8 May 2024, at 03:45, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> >
> > On Tue, May 7, 2024 at 8:01 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >>
> >> On Wed, May 08, 2024 at 12:10:45AM +0200, KP Singh wrote:
> >>> [...]
> >>> +/**
> >>> + * 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)
> >>> + 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;
> >>> +}
> >>
> >> First of all: patches 1-4 are great. They have a measurable performance
> >> benefit; let's get those in.
> >>
> >> But here I come to patch 5 where I will suggest the exact opposite of
> >> what Paul said in v9 for patch 5. :P
> >
> > For those looking up v9 of the patchset, you'll be looking for patch
> > *4*, not patch 5, as there were only four patches in the v9 series.
> > Patch 4/5 in the v10 series is a new addition to the stack.
> >
> > Beyond that, I'm guessing you are referring to my comment regarding
> > bpf_lsm_toggle_hook() Kees? The one that starts with "More ugh. If
> > we are going to solve things this way ..."?
> >
> >> I don't want to have a global function that can be used to disable LSMs.
> >> We got an entire distro (RedHat) to change their SELinux configurations
> >> to get rid of CONFIG_SECURITY_SELINUX_DISABLE (and therefore
> >> CONFIG_SECURITY_WRITABLE_HOOKS), via commit f22f9aaf6c3d ("selinux:
> >> remove the runtime disable functionality"). We cannot reintroduce that,
> >> and I'm hoping Paul will agree, given this reminder of LSM history. :)
> >>
> >> Run-time hook changing should be BPF_LSM specific, if it exists at all.
>
>
> One idea here is that only LSM hooks with default_state = false can be toggled.
>
> This would also any ROPs that try to abuse this function. Maybe we can call "default_disabled" .toggleable (or dynamic)
>
> and change the corresponding LSM_INIT_TOGGLEABLE. Kees, Paul, this may be a fair middle ground?
>
> Something like:
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 4bd1d47bb9dc..5c0918ed6b80 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -117,7 +117,7 @@ struct security_hook_list {
> struct lsm_static_call *scalls;
> union security_list_options hook;
> const struct lsm_id *lsmid;
> - bool default_enabled;
> + bool toggleable;
> } __randomize_layout;
>
> /*
> @@ -168,14 +168,18 @@ static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
> { \
> .scalls = static_calls_table.NAME, \
> .hook = { .NAME = HOOK }, \
> - .default_enabled = true \
> + .toggleable = false \
> }
>
> -#define LSM_HOOK_INIT_DISABLED(NAME, HOOK) \
> +/*
> + * 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 }, \
> - .default_enabled = false \
> + .toggleable = true \
> }
>
> extern char *lsm_names;
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index ed864f7430a3..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_DISABLED(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 b3a92a67f325..a89eb8fe302b 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -407,7 +407,8 @@ 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;
> - if (hl->default_enabled)
> + /* Toggleable hooks are inactive by default */
> + if (!hl->toggleable)
> static_branch_enable(scall->active);
> return;
> }
> @@ -901,6 +902,9 @@ int security_toggle_hook(void *hook_addr, bool state)
> int i;
>
> for (i = 0; i < num_entries; i++) {
> + if (!scalls[i].hl->toggleable)
> + continue;
> +
> if (!scalls[i].hl)
> continue;
Yeah, I like this! It's a routine that is walking read-only data to make
the choice, and it's specific to a pre-defined characteristic that an
LSM would need to opt into. My concerns are addressed! Thanks! :)
--
Kees Cook
