On Wed, 2024-02-21 at 17:25 +0100, Benjamin Tissoires wrote:
[...]
> @@ -11973,6 +12006,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
> if (ret)
> return ret;
> break;
> + case KF_ARG_PTR_TO_TIMER:
> + /* FIXME: should we do anything here? */
> + break;
I think that here it is necessary to enforce that R1
is PTR_TO_MAP_VALUE and that it points to the timer field of the map value.
As is, the following program leads to in-kernel page fault when
printing verifier log:
--- 8< ----------------------------
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 = 1;
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((void *)&arr_timer, // note incorrrect pointer type!
cb_sleepable);
bpf_timer_start(arr_timer, 0, 0);
return 0;
}
---------------------------- >8 ---
I get the page fault when doing:
$ ./veristat -l7 -vvv -f test_sleepable timer.bpf.o
[ 21.014886] BUG: kernel NULL pointer dereference, address: 0000000000000060
...
[ 21.015780] RIP: 0010:print_reg_state (kernel/bpf/log.c:715)
And here is a relevant fragment of print_reg_state():
713 if (type_is_map_ptr(t)) {
714 if (reg->map_ptr->name[0])
715 verbose_a("map=%s", reg->map_ptr->name);
716 verbose_a("ks=%d,vs=%d",
717 reg->map_ptr->key_size,
718 reg->map_ptr->value_size);
719 }
The error is caused by reg->map_ptr being NULL.
The code in check_kfunc_args() allows anything in R1,
including registers for which type is not pointer to map and reg->map_ptr is NULL.
When later the check_kfunc_call() is done it does push_callback_call():
12152 err = push_callback_call(env, insn, insn_idx, meta.subprogno,
12153 set_timer_callback_state);
Which calls set_timer_callback_state(), that sets bogus state for R{1,2,3}:
9683 static int set_timer_callback_state(...)
9684 {
9685 struct bpf_map *map_ptr = caller->regs[BPF_REG_1].map_ptr;
9687
9688 /* bpf_timer_set_callback(struct bpf_timer *timer, void *callback_fn);
9689 * callback_fn(struct bpf_map *map, void *key, void *value);
9690 */
9691 callee->regs[BPF_REG_1].type = CONST_PTR_TO_MAP;
9692 __mark_reg_known_zero(&callee->regs[BPF_REG_1]);
9693 callee->regs[BPF_REG_1].map_ptr = map_ptr;
^^^^^^^^^
This is NULL!
