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

Erik Joelsson erik.joelsson at oracle.com
Fri Sep 7 17:30:42 UTC 2018

Hello again,

Resurrecting this review. It is now part of proposed JEP and some 
details have changed. The alternate jvm variant is now called 
"nonspeculative", and so is activated with the command line flag 
"-nonspeculative". This is all in line with the proposed JEP.

Differences from last webrev are:

* The name of the new JVM variant
* Removed an unused AC_SUBST

Webrev: http://cr.openjdk.java.net/~erikj/8202384/webrev.07/index.html


On 2018-06-11 23:29, Magnus Ihse Bursie wrote:
> On 2018-06-11 22:42, Erik Joelsson wrote:
>> Hello,
>> Based on the discussion here, I have reverted back to something more 
>> similar to webrev.02, but with a few changes. Mainly fixing a bug 
>> that caused JVM_FEATURES_hardened to not actually be the same as for 
>> server (if you have custom additions in configure). I also added a 
>> check so that configure fails if you try to enable either variant 
>> hardened or feature no-speculative-cti and the flags aren't available.
>> Webrev: http://cr.openjdk.java.net/~erikj/8202384/webrev.05/index.html
>  Looks good to me. Thanks for all the effort!
> /Magnus
>> /Erik
>> On 2018-06-11 00:10, 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!
>>> 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.
>>> 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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