Re: [PATCH bpf-next v2 5/6] bpf: teach the verifier to enforce css_iter and process_iter in RCU CS

Chuyi Zhou
2023-09-14T08:56:39Z

在 2023/9/13 21:53, Chuyi Zhou 写道:
> Hello.
>
> 在 2023/9/12 15:01, Chuyi Zhou 写道:
>> css_iter and process_iter should be used in rcu section. Specifically, in
>> sleepable progs explicit bpf_rcu_read_lock() is needed before use these
>> iters. In normal bpf progs that have implicit rcu_read_lock(), it's OK to
>> use them directly.
>>
>> This patch checks whether we are in rcu cs before we want to invoke
>> bpf_iter_process_new and bpf_iter_css_{pre, post}_new in
>> mark_stack_slots_iter(). If the rcu protection is guaranteed, we would
>> let st->type = PTR_TO_STACK | MEM_RCU. is_iter_reg_valid_init() will
>> reject if reg->type is UNTRUSTED.
>
> I use the following BPF Prog to test this patch:
>
> SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
> int iter_task_for_each_sleep(void *ctx)
> {
>     struct task_struct *task;
>     struct task_struct *cur_task = bpf_get_current_task_btf();
>
>     if (cur_task->pid != target_pid)
>         return 0;
>     bpf_rcu_read_lock();
>     bpf_for_each(process, task) {
>         bpf_rcu_read_unlock();
>         if (task->pid == target_pid)
>             process_cnt += 1;
>         bpf_rcu_read_lock();
>     }
>     bpf_rcu_read_unlock();
>     return 0;
> }
>
> Unfortunately, we can pass the verifier.
>
> Then I add some printk-messages before setting/clearing state to help
> debug:
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index d151e6b43a5f..35f3fa9471a9 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1200,7 +1200,7 @@ static int mark_stack_slots_iter(struct
> bpf_verifier_env *env,
>                 __mark_reg_known_zero(st);
>                 st->type = PTR_TO_STACK; /* we don't have dedicated reg
> type */
>                 if (is_iter_need_rcu(meta)) {
> +                       printk("mark reg_addr : %px", st);
>                         if (in_rcu_cs(env))
>                                 st->type |= MEM_RCU;
>                         else
> @@ -11472,8 +11472,8 @@ static int check_kfunc_call(struct
> bpf_verifier_env *env, struct bpf_insn *insn,
>                         return -EINVAL;
>                 } else if (rcu_unlock) {
>                         bpf_for_each_reg_in_vstate(env->cur_state,
> state, reg, ({
> +                               printk("clear reg_addr : %px MEM_RCU :
> %d PTR_UNTRUSTED : %d\n ", reg, reg->type & MEM_RCU, reg->type &
> PTR_UNTRUSTED);
>                                 if (reg->type & MEM_RCU) {
> -                                       printk("clear reg addr : %lld",
> reg);
>                                         reg->type &= ~(MEM_RCU |
> PTR_MAYBE_NULL);
>                                         reg->type |= PTR_UNTRUSTED;
>                                 }
>
>
> The demsg log:
>
> [  393.705324] mark reg_addr : ffff88814e40e200
>
> [  393.706883] clear reg_addr : ffff88814d5f8000 MEM_RCU : 0
> PTR_UNTRUSTED : 0
>
> [  393.707353] clear reg_addr : ffff88814d5f8078 MEM_RCU : 0
> PTR_UNTRUSTED : 0
>
> [  393.708099] clear reg_addr : ffff88814d5f80f0 MEM_RCU : 0
> PTR_UNTRUSTED : 0
> ....
> ....
>
> I didn't see ffff88814e40e200 is cleared as expected because
> bpf_for_each_reg_in_vstate didn't find it.
>
> It seems when we are doing bpf_read_unlock() in the middle of iteration
> and want to clearing state through bpf_for_each_reg_in_vstate, we can
> not find the previous reg which we marked MEM_RCU/PTR_UNTRUSTED in
> mark_stack_slots_iter().
>

bpf_get_spilled_reg will skip slots if they are not STACK_SPILL, but in
mark_stack_slots_iter() we has marked the slots *STACK_ITER*

With the following change, everything seems work OK.

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index a3236651ec64..83c5ecccadb4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -387,7 +387,7 @@ struct bpf_verifier_state {

#define bpf_get_spilled_reg(slot, frame) \
(((slot < frame->allocated_stack / BPF_REG_SIZE) && \
- (frame->stack[slot].slot_type[0] == STACK_SPILL)) \
+ (frame->stack[slot].slot_type[0] == STACK_SPILL ||
frame->stack[slot].slot_type[0] == STACK_ITER)) \
? &frame->stack[slot].spilled_ptr : NULL)

I am not sure whether this would harm some logic implicitly when using
bpf_get_spilled_reg/bpf_for_each_spilled_reg in other place. If so,
maybe we should add a extra parameter to control the picking behaviour.

#define bpf_get_spilled_reg(slot, frame, stack_type)
\
(((slot < frame->allocated_stack / BPF_REG_SIZE) && \
(frame->stack[slot].slot_type[0] == stack_type)) \
? &frame->stack[slot].spilled_ptr : NULL)

Thanks.