Message-ID: <mafs0ecrbmzzh.fsf@kernel.org>
Date: Fri, 10 Oct 2025 01:12:18 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: "Yanjun.Zhu" <yanjun.zhu@...ux.dev>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,  Pratyush Yadav
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Subject: Re: [PATCH v3 19/30] liveupdate: luo_sysfs: add sysfs state monitoring

On Thu, Oct 09 2025, Yanjun.Zhu wrote:

> On 10/9/25 10:04 AM, Pasha Tatashin wrote:
>> On Thu, Oct 9, 2025 at 11:35 AM Zhu Yanjun <yanjun.zhu@...ux.dev> wrote:
>>>
>>> 在 2025/10/9 5:01, Pasha Tatashin 写道:
>>>>>> Because the window of kernel live update is short, it is difficult to statistics
>>>>>> how many times the kernel is live updated.
>>>>>>
>>>>>> Is it possible to add a variable to statistics the times that the kernel is live
>>>>>> updated?
>>>>> The kernel doesn't do the live update on its own. The process is driven
>>>>> and sequenced by userspace. So if you want to keep statistics, you
>>>>> should do it from your userspace (luod maybe?). I don't see any need for
>>>>> this in the kernel.
>>>>>
>>>> One use case I can think of is including information in kdump or the
>>>> backtrace warning/panic messages about how many times this machine has
>>>> been live-updated. In the past, I've seen bugs (related to memory
>>>> corruption) that occurred only after several kexecs, not on the first
>>>> one. With live updates, especially while the code is being stabilized,
>>>> I imagine we might have a similar situation. For that reason, it could
>>>> be useful to have a count in the dmesg logs showing how many times
>>>> this machine has been live-updated. While this information is also
>>>> available in userspace, it would be simpler for kernel developers
>>>> triaging these issues if everything were in one place.

Hmm, good point.

>>> I’m considering this issue from a system security perspective. After the
>>> kernel is automatically updated, user-space applications are usually
>>> unaware of the change. In one possible scenario, an attacker could
>>> replace the kernel with a compromised version, while user-space
>>> applications remain unaware of it — which poses a potential security risk.

Wouldn't signing be the way to avoid that? Because if the kernel is
compromised then it can very well fake the reboot count as well.

>>>
>>> To mitigate this, it would be useful to expose the number of kernel
>>> updates through a sysfs interface, so that we can detect whether the
>>> kernel has been updated and then collect information about the new
>>> kernel to check for possible security issues.
>>>
>>> Of course, there are other ways to detect kernel updates — for example,
>>> by using ftrace to monitor functions involved in live kernel updates —
>>> but such approaches tend to have a higher performance overhead. In
>>> contrast, adding a simple update counter to track live kernel updates
>>> would provide similar monitoring capability with minimal overhead.
>> Would a print during boot, i.e. when we print that this kernel is live
>> updating, we could include the number, work for you? Otherwise, we
>> could export this number in a debugfs.
> Since I received a notification that my previous message was not sent
> successfully, I am resending it.
>
> IMO, it would be better to export this number via debugfs. This approach reduces
> the overhead involved in detecting a kernel live update.
> If the number is printed in logs instead, the overhead would be higher compared
> to using debugfs.

Yeah, debugfs sounds fine. No ABI at least.

-- 
Regards,
Pratyush Yadav

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