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

Magnus Ihse Bursie magnus.ihse.bursie at oracle.com
Mon Jun 11 08:19:24 UTC 2018

On 2018-06-11 09:38, David Holmes wrote:
> 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.

With Erik's original proposal (webrev.02), you will, by default, get a 
hotspot "server" JVM variant that is identical to what you got without 
the patch. This should definitely cover your case.

You will also get all the non-hotspot libraries built as hardened. You 
*can* get the JDK libraries built non-hardened, by removing the 
${$2NO_SPECULATIVE_CTI_CFLAGS} from the line 

As I said, I believe this is enough to support that case.

> 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.
AFAIU, these compiler flags has received extensive testing inside 
Oracle. It is also part of a global, high-visibility project, where key 
players have had time to prepare for handling the issues ahead of the 
public awareness of the exploits. *And* it's been almost half a year 
since the Spectre exploit was made public.

I have much more faith in enabling these flags than I'd have for e.g. 
upgrading to a newer version of Solaris Studio. :-)

>> 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.

Well, now *I* am confused. ;-)

Let's separarate two components: hotspot, and the rest of the native 
code (the "JDK libraries").

For hotspot, the following holds:
* You can enable or disable no-speculative-cti as a feature on the 
configure command line, by "--with-jvm-features=no-speculative-cti" (to 
enable) or "--with-jvm-features=-no-speculative-cti" (to disable). This 
change applies to *all* JVM variants that are built; there is currently 
no command-line support for enabling or disabling features on a 
per-JVM-variant level. (There's no real hinderance to doing so; it's 
just not yet implemented).
* Erik defines a new JVM variant, which is identical to server, but has 
also no-speculative-cti enabled. This is not built by default, but can 
be enabled by --with-jvm-variants=server,hardened. Oracle will build 
OpenJDK this way.

If you do not want hardening, you just do not build the "hardened" 
variant, and you do not add the no-speculative-cti feature.

If you want hardening all over the line (and no runtime user choice), 
you add --with-jvm-feature=no-speculative-cti.


Erik's second comment "We don't see the point in giving the choice on 
the JDK libraries ..." applies not to Hotspot, but to the rest of the 
native libraries (the "JDK libraries"). Here, we will just add the 
Spectre mitigation flags, without a user runtime choice of using 
hardened or non-hardened libraries. The reason for this is that the 
hardening did not have a measurable performance impact.

The builder of the JDK still has the ability to build the JDK libraries 
without the hardening flags, but that will require modifying the 
configure script, just as the case is today if the builder should wish 
to disable any other of all the flags we enable by default.


> David
> -----
>> 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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