RFR: 8013527: calling MethodHandles.lookup on itself leads to errors

[Mandy Chung]

On Wed, 3 Feb 2021 10:52:41 GMT, Johannes Kuhn <github.com+652983+DasBrain at openjdk.org> wrote:

>> JDK-8013527: calling MethodHandles.lookup on itself leads to errors
>> JDK-8257874: MethodHandle injected invoker doesn't have necessary private access
>> 
>> Johannes Kuhn is also a contributor to this patch.
>> 
>> A caller-sensitive method can behave differently depending on the identity
>> of its immediate caller.  If a method handle for a caller-sensitive method is
>> requested, this resulting method handle behaves as if it were called from an
>> instruction contained in the lookup class.  The current implementation injects
>> a trampoline class (aka the invoker class) which is the caller class invoking
>> such caller-sensitive method handle.  It works in all CSMs except `MethodHandles::lookup`
>> because the caller-sensitive behavior depends on the module of the caller class,
>> the class loader of the caller class, the accessibility of the caller class, or
>> the protection domain of the caller class.  The invoker class is a hidden class
>> defined in the same runtime package with the same protection domain as the
>> lookup class, which is why the current implementation works for all CSMs except
>> `MethodHandles::lookup` which uses the caller class as the lookup class.
>> 
>> Two issues with current implementation:
>> 1.  The invoker class only has the package access as the lookup class.  It cannot
>>     access private members of the lookup class and its nest members.
>> 
>>     The fix is to make the invoker class as a nestmate of the lookup class.
>> 
>> 2.  `MethodHandles::lookup` if invoked via a method handle produces a `Lookup`
>>     object of an injected invoker class which is a bug.
>> 
>> There are two alternatives:
>> - define the invoker class with the lookup class as the class data such that
>>   `MethodHandles::lookup` will get the caller class from the class data if
>>   it's the injected invoker
>> - Johannes' proposal [1]: detect if an alternate implementation with an additional
>>   trailing caller class parameter is present, use the alternate implementation
>>   and bind the method handle with the lookup class as the caller class argument.
>> 
>> There has been several discussions on the improvement to support caller sensitive
>> methods for example the calling sequences and security implication.   I have
>> looked at how each CSM uses the caller class.  The second approach (i.e. 
>> defining an alternate implementation for a caller-sensitive method taking
>> an additional caller class parameter) does not work for non-static non-final
>> caller-sensitive method.  In addition, it is not ideal to pollute the source
>> code to provide an alternatve implementation for all 120+ caller-sensitive methods
>> whereas the injected invoker works for all except `MethodHandles::lookup`.
>> 
>> I propose to use both approaches.   We can add an alternative implementation for
>> a caller-sensitive method when desirable such as `MethodHandles::lookup` in
>> this PR.  For the injected invoker case, one could extract the original lookup
>> class from class data if needed.
>> 
>> test/jdk/jdk/internal/reflect/CallerSensitive/CheckCSM.java ensures that
>> no new non-static non-final caller-sensitive method will be added to the JDK.
>> I extend this test to catch that non-static non-final caller-sensitive method
>> cannot have the alternate implementation taking the additional caller class
>> parameter.
>> 
>> This fix for JDK-8013527 is needed by the prototype for JDK-6824466 I'm working on.
>> 
>> [1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-January/073184.html
>
> src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java line 1205:
> 
>> 1203:             Class<?> invokerClass = new Lookup(targetClass)
>> 1204:                     .makeHiddenClassDefiner(name, INJECTED_INVOKER_TEMPLATE, Set.of(NESTMATE))
>> 1205:                     .defineClass(true, targetClass);
> 
> Using the target class directly could lead to some unintended problems.
> 
> An attacker can define it's own hidden class as nestmate and with a name ending in `$$InjectedInvoker`.  
> This allows the attacker to "teleport" into a nestmate with full privileges.  
> An attacker could then access `MethodHandles.classData`.
> 
> Suggested remedy: Create a holder that is only visible to `java.lang.invoke`:
> 
> /* package-private */ static class OriginalCallerHolder {
>     final Class<?> originalCaller;
>     OriginalCallerHolder(Class<?> originalCaller) {
>         this.originalCaller = originalCaller;
>     }
> }
> 
> As this type is only visible inside `java.lang.invoke`, it can't be created without hacking into `java.lang.invoke`, at which point all bets are off anyway.
> 
> (A previous commit was even more dangerous, as you can force `jlr.Proxy` to inject a class into your package with a `null`-PD)

Only `Lookup` with the original access can access `MethodHandles.classData`.   A hidden class `HC$$InjectedInvoker/0x1234` can access private members of another class `C` in the same nest but not `C`'s class data.

-------------

PR: https://git.openjdk.java.net/jdk/pull/2367