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));
[...]
