Re: [PATCH v2 1/2] rust_binder: check ownership before using vma

[Jann Horn <jannh@google.com>]

On Mon, Mar 2, 2026 at 6:18 PM Carlos Llamas <cmllamas@google.com> wrote:
> On Wed, Feb 18, 2026 at 11:53:26AM +0000, Alice Ryhl wrote:
> > When installing missing pages (or zapping them), Rust Binder will look
> > up the vma in the mm by address, and then call vm_insert_page (or
> > zap_page_range_single). 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.
>
> This all makes sense to me. What I'm missing though is why not reject
> VM_WRITE mappings all together? Is there a downside or something that
> prevents us from setting this check?

You could, and it would probably do the job (assuming that you check
for VM_MAYWRITE instead of VM_WRITE), but I think it'd be more of a
surface-level mitigation than a robust safety check - in my opinion, a
robust check should, at a minimum, confirm that the VMA being accessed
belongs to the right driver, because other drivers might do random
things you don't expect in their own VMAs. (For example, it wouldn't
protect against interaction with a driver like C binder which reads
PTEs back out of the VMA in binder_page_lookup(), makes assumptions
about what kinds of pages that yields, and writes into those pages.) A
driver should not be touching VMAs it doesn't own.