CVE-2026-43434: rust_binder: check ownership before using vma

In the Linux kernel, the following vulnerability has been resolved:

rustbinder: check ownership before using vma

When installing missing pages (or zapping them), Rust Binder will look up the vma in the mm by address, and then call vminsertpage (or zappagerangesingle). However, if the vma is closed and replaced with a different vma at the same address, this can lead to Rust Binder installing pages into the wrong vma.

By installing the page into a writable vma, it becomes possible to write to your own binder pages, which are normally read-only. Although you're not supposed to be able to write to those pages, the intent behind the design of Rust Binder is that even if you get that ability, it should not lead to anything bad. Unfortunately, due to another bug, that is not the case.

To fix this, store a pointer in vmprivatedata and check that the vma returned by vmalookup() has the right vmops and vmprivatedata before trying to use the vma. This should ensure that Rust Binder will refuse to interact with any other VMA. The plan is to introduce more vma abstractions to avoid this unsafe access to vmops and vmprivatedata, but for now let's start with the simplest possible fix.

C Binder performs the same check in a slightly different way: it provides a vmops->close that sets a boolean to true, then checks that boolean after calling vmalookup(), but this is more fragile than the solution in this patch. (We probably still want to do both, but the vmops->close callback will be added later as part of the follow-up vma API changes.)

It's still possible to remap the vma so that pages appear in the right vma, but at the wrong offset, but this is a separate issue and will be fixed when Rust Binder gets a vmops->close callback.