Re: [PATCH RFC bpf-next v3 05/16] bpf/verifier: add bpf_timer as a kfunc capable type

[Eduard Zingerman]

On Wed, 2024-02-21 at 17:25 +0100, Benjamin Tissoires wrote:

[...]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f81c799b2c80..2b11687063ff 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5444,6 +5444,26 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
>  					return -EACCES;
>  				}
>  				break;
> +			case BPF_TIMER:
> +				/* FIXME: kptr does the above, should we use the same? */

I don't think so.
Basically this allows double word reads / writes from timer address,
which probably should not be allowed.

The ACCESS_DIRECT is passed to check_map_access() from
check_mem_access() and I don't see points where check_mem_access()
call would be triggered for pointer parameter of kfunc
(unless it is accompanied by a size parameter).

I tried the following simple program and it verifies fine:

    struct elem {
    	struct bpf_timer t;
    };

    struct {
    	__uint(type, BPF_MAP_TYPE_ARRAY);
    	__uint(max_entries, 2);
    	__type(key, int);
    	__type(value, struct elem);
    } array SEC(".maps");

    int bpf_timer_set_sleepable_cb
      (struct bpf_timer *timer,
       int (callback_fn)(void *map, int *key, struct bpf_timer *timer))
      __ksym __weak;

    static int cb_sleepable(void *map, int *key, struct bpf_timer *timer)
    {
    	return 0;
    }

    SEC("fentry/bpf_fentry_test5")
    int BPF_PROG2(test_sleepable, int, a)
    {
    	struct bpf_timer *arr_timer;
    	int array_key = ARRAY;

    	arr_timer = bpf_map_lookup_elem(&array, &array_key);
    	if (!arr_timer)
    		return 0;
    	bpf_timer_init(arr_timer, &array, CLOCK_MONOTONIC);

    	bpf_timer_set_sleepable_cb(arr_timer, cb_sleepable);
    	bpf_timer_start(arr_timer, 0, 0);

    	return 0;
    }

(in general, it would be easier to review if there were some test
 cases to play with).

> +				if (src != ACCESS_DIRECT) {
> +					verbose(env, "bpf_timer cannot be accessed indirectly by helper\n");
> +					return -EACCES;
> +				}
> +				if (!tnum_is_const(reg->var_off)) {
> +					verbose(env, "bpf_timer access cannot have variable offset\n");
> +					return -EACCES;
> +				}
> +				if (p != off + reg->var_off.value) {
> +					verbose(env, "bpf_timer access misaligned expected=%u off=%llu\n",
> +						p, off + reg->var_off.value);
> +					return -EACCES;
> +				}
> +				if (size != bpf_size_to_bytes(BPF_DW)) {
> +					verbose(env, "bpf_timer access size must be BPF_DW\n");
> +					return -EACCES;
> +				}
> +				break;
>  			default:
>  				verbose(env, "%s cannot be accessed directly by load/store\n",
>  					btf_field_type_name(field->type));

[...]