RFR: JDK-8202384: Introduce altserver jvm variant with speculative execution disabled

David Holmes david.holmes at oracle.com
Mon Jun 11 07:38:14 UTC 2018

Hi Magnus,

On 11/06/2018 5:10 PM, Magnus Ihse Bursie wrote:
> On 2018-06-08 23:50, Erik Joelsson wrote:
>> On 2018-06-07 17:30, David Holmes wrote:
>>> On 8/06/2018 6:11 AM, Erik Joelsson wrote:
>>>> I just don't think the extra work is warranted or should be 
>>>> prioritized at this point. I also cannot think of a combination of 
>>>> options required for what you are suggesting that wouldn't be 
>>>> confusing to the user. If someone truly feels like these flags are 
>>>> forced on them and can't live with them, we or preferably that 
>>>> person can fix it then. I don't think that's dictatorship. OpenJDK 
>>>> is still open source and anyone can contribute.
>>> I don't see why --enable-hardened-jdk and --enable-hardened-hotspot 
>>> to add to the right flags would be either complicated or confusing.
>> For me the confusion surrounds the difference between 
>> --enable-hardened-hotspot and --with-jvm-variants=server, hardened and 
>> making the user understand it. But sure, it is doable. Here is a new 
>> webrev with those two options as I interpret them. Here is the help text:
>>  --enable-hardened-jdk   enable hardenening compiler flags for all jdk
>>                           libraries (except the JVM), typically disabling
>>                           speculative cti. [disabled]
>>  --enable-hardened-hotspot
>>                           enable hardenening compiler flags for 
>> hotspot (all
>>                           jvm variants), typically disabling 
>> speculative cti.
>>                           To make hardening of hotspot a runtime choice,
>>                           consider the "hardened" jvm variant instead 
>> of this
>>                           option. [disabled]
>> Note that this changes the default for jdk libraries to not enable 
>> hardening unless the user requests it.
>> Webrev: http://cr.openjdk.java.net/~erikj/8202384/webrev.04/
> Hold it, hold it! I'm not sure how we ended up here, but I don't like it 
> at all. :-(
> I think Eriks initial patch is much better than this. Some arguments in 
> random order to defend this position:
> 1) Why should we have a configure option to disable security relevant 
> flags for the JDK, if there has been no measured negative effect? We 
> don't do this for any other compiler flags, especially not security 
> relevant ones!
> I've re-read the entire thread to see if I could understand what could 
> possibly motivate this, but the only thing I can find is David Holmes 
> vague fear that these flags would not be well-tested enough. Let me 
> counter with my own vague guesses: I believe the spectre mitigation 
> methods to have been fully and properly tested, since they are 
> rolled-out massively on all products. And let me complement with my own 
> fear: the PR catastrophe if OpenJDK were *not* built with spectre 
> mitigations, and someone were to exploit that!

All I'm looking for is the ability to select whether you can build with 
or without this "hardening". The default OpenJDK build can of course 
churn out a "hardened" implementation. Anyone who opts out of that is on 
their own.

I don't share your faith or confidence in the quality of any software 
rushed out in a fairly short space of time. Prudence, if nothing else, 
says you should be able to not build this way IMHO.

> In fact, I could even argue that "server" should be hardened *by 
> default*, and that we should instead introduce a non-hardened JVM named 
> something akin to "quick-but-dangerous-server" instead. But I realize 
> that a 25% performance hit is hard to swallow, so I won't push this agenda.
> 2) It is by no means clear that "--enable-hardened-jdk" does not harden 
> all aspects of the JDK! If we should keep the option (which I definitely 
> do not think we should!) it should be renamed to 
> "--enable-hardened-libraries", or something like that. And it should be 
> on by default, so it should be a "--disabled-hardened-jdk-libraries".
> Also, the general-sounding name "hardened" sounds like it might 
> encompass more things than it does. What if I disabled a hardened jdk 
> build, should I still get stack banging protection? If so, you need to 
> move a lot more security-related flags to this option. (And, just to be 
> absolutely clear: I don't think you should do that.)
> 3) Having two completely different ways of turning on Spectre protection 
> for hotspot is just utterly confusing! This was a perfect example of how 
> to use the JVM features, just as in the original patch.

Okay. I have had some confusion over "features" versus "variants" based 
on Eriks earlier comments. Erik's email from June 6 first states:

"I agree, and you sort of can. By adding the jvm feature 
"no-speculative-cti" to any jvm variant, you get the flags."

but then later said:

"We don't see the point in giving the choice on the JDK libraries ..."

by which I now think he meant not giving the choice at the VM variant 
level, but I mistook it for meaning at the "feature" level. Hence I came 
back with the two flags suggestion. If we can already select features 
arbitrarily at configure time then this is all addressed already.

Apologies for the confusion.


> If you want to have spectre mitigation enabled for both server and 
> client, by default, you would just need to run "configure 
> --with-jvm-variants=server,client 
> --with-jvm-features=no-speculative-cti", which will enable that feature 
> for all variants. That's not really hard *at all* for anyone building 
> OpenJDK. And it's way clearer what will happen, than a 
> --enable-hardened-hotspot.
> 4) If you are a downstream provider building OpenJDK and you are dead 
> set on not including Spectre mitigations in the JDK libraries, despite 
> being shown to have no negative effects, then you can do just as any 
> other downstream user with highly specialized requirements, and patch 
> the source. I have no sympathies for this; I can't stop it but I don't 
> think there's any reason for us to complicate the code to support this 
> unlikely case.
> So, to recap, I think the webrev as published in 
> http://cr.openjdk.java.net/~erikj/8202384/webrev.02/ (with "altserver" 
> renamed to "hardened") is the way to go.
> /Magnus
>> /Erik

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