[PATCH v5 4/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[Jiri Olsa]

On Thu, Sep 28, 2023 at 10:24:09PM +0200, KP Singh wrote:

SNIP

> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index e97aeda3a86b..df9699bce372 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 = {
> @@ -514,7 +515,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
>  {
>  	enum bpf_tramp_prog_type kind;
>  	struct bpf_tramp_link *link_exiting;
> -	int err = 0;
> +	int err = 0, num_lsm_progs = 0;
>  	int cnt = 0, i;
>  
>  	kind = bpf_attach_type_to_tramp(link->link.prog);
> @@ -545,8 +546,14 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
>  			continue;
>  		/* prog already linked */
>  		return -EBUSY;
> +
> +		if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> +			num_lsm_progs++;

this looks wrong, it's never reached.. seems like we should add separate
hlist_for_each_entry loop over trampoline's links for this check/init of
num_lsm_progs ?

jirka

>  	}
>  
> +	if (!num_lsm_progs && link->link.prog->type == BPF_PROG_TYPE_LSM)
> +		bpf_lsm_toggle_hook(tr->func.addr, true);
> +
>  	hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
>  	tr->progs_cnt[kind]++;
>  	err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> @@ -569,8 +576,10 @@ int bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_trampoline
>  
>  static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_trampoline *tr)
>  {
> +	struct bpf_tramp_link *link_exiting;
>  	enum bpf_tramp_prog_type kind;
> -	int err;
> +	bool lsm_link_found = false;
> +	int err, num_lsm_progs = 0;
>  
>  	kind = bpf_attach_type_to_tramp(link->link.prog);
>  	if (kind == BPF_TRAMP_REPLACE) {
> @@ -580,8 +589,24 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
>  		tr->extension_prog = NULL;
>  		return err;
>  	}
> +
> +	if (link->link.prog->type == BPF_PROG_TYPE_LSM) {
> +		hlist_for_each_entry(link_exiting, &tr->progs_hlist[kind],
> +				     tramp_hlist) {
> +			if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> +				num_lsm_progs++;
> +
> +			if (link_exiting->link.prog == link->link.prog)
> +				lsm_link_found = true;
> +		}
> +	}
> +
>  	hlist_del_init(&link->tramp_hlist);
>  	tr->progs_cnt[kind]--;
> +
> +	if (lsm_link_found && num_lsm_progs == 1)
> +		bpf_lsm_toggle_hook(tr->func.addr, false);
> +
>  	return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
>  }
>  
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index cfaf1d0e6a5f..1957244196d0 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];
> +		scalls = h->scalls;
> +		if (h->hook.lsm_callback == addr)
> +			continue;
> +
> +		for (j = 0; j < MAX_LSM_COUNT; j++) {
> +			if (scalls[j].hl != h)
> +				continue;
> +			if (value)
> +				static_branch_enable(scalls[j].active);
> +			else
> +				static_branch_disable(scalls[j].active);
> +		}
> +	}
> +}
> diff --git a/security/security.c b/security/security.c
> index c2c2cf6b711f..d1ee72e563cc 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++;
> -- 
> 2.42.0.582.g8ccd20d70d-goog
> 
>