Re: [PATCH v2 bpf-next 5/7] bpf: Consider non-owning refs to refcounted nodes RCU protected

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

On 8/21/23 12:33 PM, Dave Marchevsky wrote:
> An earlier patch in the series ensures that the underlying memory of
> nodes with bpf_refcount - which can have multiple owners - is not reused
> until RCU 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.
> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
>   include/linux/bpf.h   |  3 ++-
>   kernel/bpf/verifier.c | 13 ++++++++++++-
>   2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index eced6400f778..12596af59c00 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.
>   	 */
>   	NON_OWN_REF		= BIT(14 + BPF_BASE_TYPE_BITS),
>   
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8db0afa5985c..55607ab30522 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -8013,6 +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
> @@ -10479,6 +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");
> @@ -10491,6 +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 the above MEM_RCU marking be done unless reg access is in
rcu critical section?

I think we still have issues for state resetting
with bpf_spin_unlock() and bpf_rcu_read_unlock(), both of which
will try to convert the reg state to PTR_UNTRUSTED.

Let us say reg state is
   PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU

(1). If hitting bpf_spin_unlock(), since MEM_RCU is in
the reg state, the state should become
   PTR_TO_BTF_ID | MEM_ALLOC | MEM_RCU
some additional code might be needed so we wont have
verifier complaints about ref_obj_id == 0.

(2). If hitting bpf_rcu_read_unlock(), the state should become
   PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF
since register access still in bpf_spin_lock() region.

Does this make sense?

> +
>   	return 0;
>   }
>   
> @@ -11328,6 +11333,11 @@ 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, "Calling bpf_rcu_read_{lock,unlock} in unnecessary rbtree callback\n");
> +			return -EACCES;
> +		}
> +
>   		if (rcu_lock) {
>   			verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
>   			return -EINVAL;
> @@ -16689,7 +16699,8 @@ 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;
>   				}