Re: BUG: unable to handle kernel paging request in bpf_dispatcher_xdp

[Yonghong Song <yhs@meta.com>]

On 12/9/22 7:20 AM, Jiri Olsa wrote:
> On Fri, Dec 09, 2022 at 02:50:55PM +0100, Jiri Olsa wrote:
>> On Fri, Dec 09, 2022 at 12:22:37PM +0100, Jiri Olsa wrote:
>>
>> SBIP
>>
>>>>>>>>>
>>>>>>>>> I'm trying to understand the severity of the issues and
>>>>>>>>> whether we need to revert that commit asap since the merge window
>>>>>>>>> is about to start.
>>>>>>>>
>>>>>>>> Jiri, Peter,
>>>>>>>>
>>>>>>>> ping.
>>>>>>>>
>>>>>>>> cc-ing Thorsten, since he's tracking it now.
>>>>>>>>
>>>>>>>> The config has CONFIG_X86_KERNEL_IBT=y.
>>>>>>>> Is it related?
>>>>>>>
>>>>>>> sorry for late reply.. I still did not find the reason,
>>>>>>> but I did not try with IBT yet, will test now
>>>>>>
>>>>>> no difference with IBT enabled, can't reproduce the issue
>>>>>>
>>>>>
>>>>> ok, scratch that.. the reproducer got stuck on wifi init :-\
>>>>>
>>>>> after I fix that I can now reproduce on my local config with
>>>>> IBT enabled or disabled.. it's something else
>>>>
>>>> I'm getting the error also when reverting the static call change,
>>>> looking for good commit, bisecting
>>>>
>>>> I'm getting fail with:
>>>>     f0c4d9fc9cc9 (tag: v6.1-rc4) Linux 6.1-rc4
>>>>
>>>> v6.1-rc1 is ok
>>>
>>> so far I narrowed it down between rc1 and rc3.. bisect got me nowhere so far
>>>
>>> attaching some more logs
>>
>> looking at the code.. how do we ensure that code running through
>> bpf_prog_run_xdp will not get dispatcher image changed while
>> it's being exetuted
>>
>> we use 'the other half' of the image when we add/remove programs,
>> but could bpf_dispatcher_update race with bpf_prog_run_xdp like:
>>
>>
>> cpu 0:                                  cpu 1:
>>
>> bpf_prog_run_xdp
>>     ...
>>     bpf_dispatcher_xdp_func
>>        start exec image at offset 0x0
>>
>>                                          bpf_dispatcher_update
>>                                                  update image at offset 0x800
>>                                          bpf_dispatcher_update
>>                                                  update image at offset 0x0
>>
>>        still in image at offset 0x0
>>
>>
>> that might explain why I wasn't able to trigger that on
>> bare metal just in qemu
> 
> I tried patch below and it fixes the issue for me and seems
> to confirm the race above.. but not sure it's the best fix
> 
> jirka
> 
> 
> ---
> diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
> index c19719f48ce0..6a2ced102fc7 100644
> --- a/kernel/bpf/dispatcher.c
> +++ b/kernel/bpf/dispatcher.c
> @@ -124,6 +124,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
>   	}
>   
>   	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);
> +	synchronize_rcu_tasks();
>   
>   	if (new)
>   		d->image_off = noff;

This might work. In arch/x86/kernel/alternative.c, we have following
code and comments. For text_poke, synchronize_rcu_tasks() might be able
to avoid concurrent execution and update.

/**
  * text_poke_copy - Copy instructions into (an unused part of) RX memory
  * @addr: address to modify
  * @opcode: source of the copy
  * @len: length to copy, could be more than 2x PAGE_SIZE
  *
  * Not safe against concurrent execution; useful for JITs to dump
  * new code blocks into unused regions of RX memory. Can be used in
  * conjunction with synchronize_rcu_tasks() to wait for existing
  * execution to quiesce after having made sure no existing functions
  * pointers are live.
  */
void *text_poke_copy(void *addr, const void *opcode, size_t len)
{
         unsigned long start = (unsigned long)addr;
         size_t patched = 0;

         if (WARN_ON_ONCE(core_kernel_text(start)))
                 return NULL;

         mutex_lock(&text_mutex);
         while (patched < len) {
                 unsigned long ptr = start + patched;
                 size_t s;

                 s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), 
len - patched);

                 __text_poke(text_poke_memcpy, (void *)ptr, opcode + 
patched, s);
                 patched += s;
         }
         mutex_unlock(&text_mutex);
         return addr;
}