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

Daniel Borkmann
2022-12-09T22:41:32Z

On 12/9/22 10:53 PM, Jiri Olsa wrote:
> On Fri, Dec 09, 2022 at 12:31:06PM -0800, Yonghong Song wrote:
>>
>>
>> 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.
>
> so my idea was that we need to ensure all the current callers of
> bpf_dispatcher_xdp_func (which should have rcu read lock, based
> on the comment in bpf_prog_run_xdp) are gone before and new ones
> execute the new image, so the next call to the bpf_dispatcher_update
> will be safe to overwrite the other half of the image

If v6.1-rc1 was indeed okay, then it looks like this may be related to
the trampoline patching for the static_call? Did it repro on v6.1-rc1
just with dbe69b299884 ("bpf: Fix dispatcher patchable function entry
to 5 bytes nop") cherry-picked?

>> /**
>> * 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;
>> }