Re: [PATCH v1 bpf-next 5/7] bpf: Consider non-owning refs to refcounted nodes RCU protected

[Yonghong Song <yonghong.song@linux.dev>]

On 8/1/23 1:36 PM, Dave Marchevsky wrote:
> The previous patch in the series ensures that the underlying memory of
> nodes with bpf_refcount - which can have multiple owners - is not reused
> until RCU Tasks Trace grace period has elapsed. This prevents
> use-after-free with non-owning references that may point to
> recently-freed memory. While RCU read lock is held, it's safe to
> dereference such a non-owning ref, as by definition RCU GP couldn't have
> elapsed and therefore underlying memory couldn't have been reused.
> 
>  From the perspective of verifier "trustedness" non-owning refs to
> refcounted nodes are now trusted only in RCU CS and therefore should no
> longer pass is_trusted_reg, but rather is_rcu_reg. Let's mark them
> MEM_RCU in order to reflect this new state.
> 
> Similarly to bpf_spin_unlock being a non-owning ref invalidation point,
> where non-owning ref reg states are clobbered so that they cannot be
> used outside of the critical section, currently all MEM_RCU regs are
> marked untrusted after bpf_rcu_read_unlock. This patch makes
> bpf_rcu_read_unlock a non-owning ref invalidation point as well,
> clobbering the non-owning refs instead of marking untrusted. In the
> future we may want to allow untrusted non-owning refs in which case we
> can remove this custom logic without breaking BPF programs as it's more
> restrictive than the default. That's a big change in semantics, though,
> and this series is focused on fixing the use-after-free in most
> straightforward way.
> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
>   include/linux/bpf.h   |  3 ++-
>   kernel/bpf/verifier.c | 17 +++++++++++++++--
>   2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index ceaa8c23287f..37fba01b061a 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -653,7 +653,8 @@ enum bpf_type_flag {
>   	MEM_RCU			= BIT(13 + BPF_BASE_TYPE_BITS),
>   
>   	/* Used to tag PTR_TO_BTF_ID | MEM_ALLOC references which are non-owning.
> -	 * Currently only valid for linked-list and rbtree nodes.
> +	 * Currently only valid for linked-list and rbtree nodes. If the nodes
> +	 * have a bpf_refcount_field, they must be tagged MEM_RCU as well.

What does 'must' here mean?

>   	 */
>   	NON_OWN_REF		= BIT(14 + BPF_BASE_TYPE_BITS),
>   
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9014b469dd9d..4bda365000d3 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -469,7 +469,8 @@ static bool type_is_ptr_alloc_obj(u32 type)
>   
>   static bool type_is_non_owning_ref(u32 type)
>   {
> -	return type_is_ptr_alloc_obj(type) && type_flag(type) & NON_OWN_REF;
> +	return type_is_ptr_alloc_obj(type) &&
> +		type_flag(type) & NON_OWN_REF;

There is no code change here.

>   }
>   
>   static struct btf_record *reg_btf_record(const struct bpf_reg_state *reg)
> @@ -8012,6 +8013,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
>   	case PTR_TO_BTF_ID | PTR_TRUSTED:
>   	case PTR_TO_BTF_ID | MEM_RCU:
>   	case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF:
> +	case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU:
>   		/* When referenced PTR_TO_BTF_ID is passed to release function,
>   		 * its fixed offset must be 0. In the other cases, fixed offset
>   		 * can be non-zero. This was already checked above. So pass
> @@ -10478,6 +10480,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
>   static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
>   {
>   	struct bpf_verifier_state *state = env->cur_state;
> +	struct btf_record *rec = reg_btf_record(reg);
>   
>   	if (!state->active_lock.ptr) {
>   		verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n");
> @@ -10490,6 +10493,9 @@ static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state
>   	}
>   
>   	reg->type |= NON_OWN_REF;
> +	if (rec->refcount_off >= 0)
> +		reg->type |= MEM_RCU;

Should we check whether the state is in rcu cs before marking MEM_RCU?

> +
>   	return 0;
>   }
>   
> @@ -11327,10 +11333,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>   		struct bpf_func_state *state;
>   		struct bpf_reg_state *reg;
>   
> +		if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) {
> +			verbose(env, "can't rcu read {lock,unlock} in rbtree cb\n");
> +			return -EACCES;
> +		}
> +
>   		if (rcu_lock) {
>   			verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
>   			return -EINVAL;
>   		} else if (rcu_unlock) {
> +			invalidate_non_owning_refs(env);

If we have both spin lock and rcu like

      bpf_rcu_read_lock()
      ...
      bpf_spin_lock()
      ...
      bpf_spin_unlock()  <=== invalidate all non_owning_refs
      ...                <=== MEM_RCU type is gone
      bpf_rcu_read_unlock()

Maybe we could fine tune here to preserve MEM_RCU after bpf_spin_unlock()?

>   			bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
>   				if (reg->type & MEM_RCU) {
>   					reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
> @@ -16679,7 +16691,8 @@ static int do_check(struct bpf_verifier_env *env)
>   					return -EINVAL;
>   				}
>   
> -				if (env->cur_state->active_rcu_lock) {
> +				if (env->cur_state->active_rcu_lock &&
> +				    !in_rbtree_lock_required_cb(env)) {
>   					verbose(env, "bpf_rcu_read_unlock is missing\n");
>   					return -EINVAL;
>   				}