RFE: support safely wrapping restricted FFM calls

[Maurizio Cimadamore]

Hi Rob,
honestly I think this all looks ok (as in: I don't think you need to do 
anything in particular). You are writing a library that will need to 
create some downcall method handles and dereference some pointers. You 
need unsafe access, so users of your library will need to pass your 
module name to the --enable-native-access flag.

It is true that clients of your library will be able to, indirectly 
(through your framework), perform unsafe operations, but _only if_ they 
have explictly trusted your library module on the command line. At that 
point they get full access to your library (and the pointers it generate).

This is not dissimilar, if you think about it, to what happens with 
method handles. E.g. if a client A creates a method handle for a method 
M that is _only_ accessible from A, it is then free to publish that 
method handle to be used by other clients, including a client B who 
might not otherwise have access to M. This is fine - method handles are 
_capabilities_ that are granted at the moment they are obtained (via a 
lookup object). Your library acts in a similar way - it creates a lot of 
unsafe stuff which is then exposed (as safely as possible, I hope :-) ), 
to a client that would not otherwise have access to such unsafe 
capabilities.

P.S.

If you _really_ want to ban clients that don't have enable native access 
supported, I propose that your API should take a MethodHandle.Lookup, 
and then you could check the module of the lookup class, and throw an 
exception if native access is not enabled:

https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/invoke/MethodHandles.Lookup.html#lookupClass()

https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Module.html#isNativeAccessEnabled()

You can even throw if the lookup doesn't have enough access:

https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/invoke/MethodHandles.Lookup.html#hasFullPrivilegeAccess()

Maurizio



On 15/11/2023 17:13, Rob Spoor wrote:
> Hello all,
>
> I'm working on a module that makes working with FFM easier; think of 
> something like JNA. For instance, it allows creating structures 
> without having to manually manage var handles etc.
>
> My module uses restricted mehods like AddressLayout.withTargetLayout 
> to support pointers. Those correctly give warnings if I don't use 
> --enable-native-access. This is where I've identified a potential 
> security risk. Native access would need to be enabled for *my* module, 
> which would allow modules that use my module to call these restricted 
> methods indirectly and without needing native access enabled 
> themselves. This means that any malicious module could piggy-back on 
> the native access that would be enabled for my module.
>
> I can implement my own access checks using the following:
>
>     StackWalker.getInstance(Set.of(Option.RETAIN_CLASS_REFERENCE))
>             .getCallerClass()
>             .getModule()
>             .isNativeAccessEnabled()
>
> However, that would mean users of my module would need to provide 
> access using two different mechanisms. I think that making some 
> existing code public could help situations like mine:
>
> * Changing the visibility of java.lang.Module.ensureNativeAccess from 
> package-private to public would allow me to check access using the 
> JVM's own mechanism, in combination with the StackWalker class to get 
> the caller (current) class and its module. Alternatively, new instance 
> method Class.ensureNativeAccess(owner, methodName) could delegate to 
> Reflection.ensureNativeAccess(this, owner, methodName) to make sure 
> that a different module couldn't be used instead, or static method 
> Class.ensureNativeAccess(currentClass, owner, methodName) could 
> delegate to Reflection.ensureNativeAccess to support null classes.
>
> * Moving jdk.internal.javac.Restricted to java.lang.foreign would 
> allow me to easily document that methods are restricted.
>
> There is an alternative in using --add-exports to access 
> jdk.internal.reflect (for Reflection.ensureNativeAccess that 
> indirectly calls Module.ensureNativeAccess) and jdk.internal.javac 
> (for Restricted), but that adds another burden on callers. In this 
> case, a burden that cannot be easily remedied using a manifest entry 
> (Enable-Native-Access).
>
>
> Kind regards,
>
> Rob