[External] : Re: Disallowing the dynamic loading of agents by default

Mon Mar 20 10:36:01 UTC 2023

Hi,

On Mon, Mar 20, 2023 at 11:11 AM Ron Pressler <ron.pressler at oracle.com>
wrote:

> Hi.
>
> The majority of serviceability tools don’t require dynamically loading an
> agent, and the majority of applications never load an agent dynamically.
>

The majority of the JDK built-in tools, I would say. What about eg. the JMC
agent?

>
> True, there are some tools that will be affected, which is why the
> decision was to introduce the flag in JDK 9 and to announce this change,
> but change the default in a later version to give tools ample time to
> prepare their users. The rationale for this change then hasn’t changed, but
> will be reiterated in a JEP (we just wanted to announce this ahead of the
> JEP to give tool authors another reminder more than six months ahead of JDK
> 21). The only change between then and now is that even fewer use cases
> require dynamically loaded agents, and so the impact is even smaller.
>

As a maintainer of one of such tools I can confidently say that this change
will either kill the tool as the ease of use will be gone or the workaround
(eg. using JAVA_TOOL_OPTIONS) will completely defeat the purpose of this
change. Having to put a flag when starting the JVM to allow dynamic loading
of agents sounds a bit nonsensical to me - it would be much easier to
directly add the agent to the JVM startup and then implement a lightweight
control protocol over socket/shared memory to enabled/disable the agent
features dynamically.

>
> It is also true that, when starting an application you don’t know that you
> *will* need to load an agent, but in most situations you know that you
> might. E.g. processes that are too critical to bring down even for deep
> maintenance (although not many of these are written in modern version of
> Java anyone) or canary services that are under trial. The relatively few
> sophisticated users who know how to write ad-hoc agents can even opt to
> enable dynamic agent loading on all their servers; these users are better
> equipped to can weigh the risks and tradeoffs involved.
>

Wouldn't having this enabled system-wide actually defeat the purpose of
having this flag? Considering that the dynamic attach can be performed only
on the same host under the same user as the target process there seems to
be a very small chance of loading agents accidentally. In the end people
would set up their systems to enabled dynamic agent loading via eg.
JAVA_TOOL_OPTIONS and we will be in the same place as before, with the
additional hurdle of setting everything up.

> Finally, some tools that require a dynamically loaded JVM TI agents, such
> as profilers that profile native code, are so tied to the VM's internals
> that the best place for them is in the JDK. If anything, the bigger problem
> is not that profilers are used too much in production, but too little,
> including less advanced ones that don’t require an agent. There is plenty
> of time to enhance the JDK’s built-in profiling capabilities ahead of
> demand.
>

I think this is an overly optimistic view. It is *much more* difficult to
enhance the JDK's built-in profiling capabilities than do the same in an
external profiling agent.

Overall, I don't seem to understand the anticipated attack vectors this
change is supposed to prevent. AFAIK, in order to do the dynamic agent load
one needs to have full access to the target process. That means that there
are more convenient and straightforward ways to do anything nefarious than
loading a JVMTI agent. Am I missing some other usages where the JVMTI agent
would actually give access to something which would be otherwise
inaccessible considering that the attacher and attachee must be on the same
host and under the same user?

Cheers,

-JB-

>
> — Ron
>
> On 20 Mar 2023, at 01:21, Andrei Pangin <andrei.pangin at gmail.com> wrote:
>
> Hi all,
>
> Serviceability has been one of the biggest Java strengths, but the
> proposed change is going to have a large negative impact on it.
>
> Disallowing dynamic agents by default means it will no longer be possible
> to attach a profiler to a running app in runtime. JFR cannot close this gap
> due to lack of capabilities modern Java profilers have (that's a separate
> topic though).
>
> When an issue happens with a live app, it's already too late to add a
> command line argument. Furthermore, it may not be even feasible to add an
> agent at startup in containerized applications. Starting profiler on demand
> from the host OS or from a sidecar is the only viable solution in these
> cases.
>
> Next, it's hard to predict beforehand what tools exactly might be useful
> for troubleshooting: e.g., one tool may be better for finding memory leaks,
> a different one for analyzing CPU performance. Adding all possible tools at
> startup does not seem a reasonable approach, especially when tools may
> conflict with each other.
>
> The most important aspect of dynamic agents is the possibility to make a
> special tool just in time for solving a particular problem. A typical
> example is to get a value of some field in a live app without dumping the
> entire 60 GB heap. Another common use case is hot patching for fixing
> trivial bugs or for adding debug logs dynamically. The prominent example is
> when the dynamic agent has proved irreplaceable aid in addressing the
> notorious log4j vulnerabilities CVE-2021-44228 and CVE-2021-45046.
>
> I would be grateful to know more about the reasons why we should give up
> all the above advantages of dynamic agents in their good and legitimate use
> cases.
>
> Thank you,
> Andrei
>
> чт, 16 мар. 2023 г. в 18:48, Ron Pressler <ron.pressler at oracle.com>:
>
>> Hi.
>>
>> In JDK 21 we intend to disallow the dynamic loading of agents by default.
>> This
>> will affect tools that use the Attach API to load an agent into a JVM
>> some time
>> after the JVM has started [1]. There is no change to any of the
>> mechanisms that
>> load an agent at JVM startup (-javaagent/-agentlib on the command line or
>> the
>> Launcher-Agent-Class attribute in the main JAR's manifest).
>>
>> This change in default behavior was proposed in 2017 as part of JEP 261
>> [2][3].
>> At that time the consensus was to switch to this default not in JDK 9 but
>> in a
>> later release to give tool maintainers sufficient time to inform their
>> users.
>> To allow the dynamic loading of agents, users will need to specify
>> -XX:+EnableDynamicAgentLoading on the command line.
>>
>> I'll post a draft JEP for review shortly.
>>
>> -- Ron
>>
>> [1]:
>> https://docs.oracle.com/en/java/javase/19/docs/api/jdk.attach/com/sun/tools/attach/package-summary.html
>> [2]: https://openjdk.org/jeps/261
>> [3]: https://mail.openjdk.org/pipermail/jigsaw-dev/2017-April/012040.html
>
>
>
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