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

[Danilo Krummrich]

On Tue Feb 17, 2026 at 3:22 PM CET, 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.
>
> To fix this, I will store a pointer in vm_private_data and check that
> the vma returned by vma_lookup() has the right vm_ops and
> vm_private_data before trying to use the vma. This should ensure that
> Rust Binder will refuse to interact with any other VMA. I will follow up
> this patch with more vma abstractions to avoid this unsafe access to
> vm_ops and vm_private_data, but for now I'd like to start with the
> simplest possible fix.

I suggest to use imperative mood instead.

> C Binder performs the same check in a slightly different way: it
> provides a vm_ops->close that sets a boolean to true, then checks that
> boolean after calling vma_lookup(), but I think this is more fragile
> than the solution in this patch. (We probably still want to do both, but
> I'll add the vm_ops->close callback with the follow-up vma API changes.)
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>

If you have a link, please add Closes: after Reported-by:.

> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
>  drivers/android/binder/page_range.rs | 78 +++++++++++++++++++++++++++---------
>  1 file changed, 58 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs
> index fdd97112ef5c8b2341e498dc3567b659f05e3fd7..90bab18961443c6e59699cb7345e41e0db80f0dd 100644
> --- a/drivers/android/binder/page_range.rs
> +++ b/drivers/android/binder/page_range.rs
> @@ -142,6 +142,27 @@ pub(crate) struct ShrinkablePageRange {
>      _pin: PhantomPinned,
>  }
>  
> +// We do not define any ops. For now, used only to check identity of vmas.
> +static BINDER_VM_OPS: bindings::vm_operations_struct = pin_init::zeroed();
> +
> +// To ensure that we do not accidentally install pages into or zap pages from the wrong vma, we
> +// check its vm_ops and private data before using it.
> +fn check_vma(vma: &virt::VmaRef, owner: *const ShrinkablePageRange) -> Option<&virt::VmaMixedMap> {
> +    // SAFETY: Just reading the vm_ops pointer of any active vma is safe.

Here and in a few other places, missing markdown.

> +    let vm_ops = unsafe { (*vma.as_ptr()).vm_ops };
> +    if !ptr::eq(vm_ops, &BINDER_VM_OPS) {
> +        return None;
> +    }
> +
> +    // SAFETY: Reading the vm_private_data pointer of a binder-owned vma is safe.
> +    let vm_private_data = unsafe { (*vma.as_ptr()).vm_private_data };
> +    if !ptr::eq(vm_private_data, owner.cast()) {
> +        return None;
> +    }
> +
> +    vma.as_mixedmap_vma()
> +}
> +
>  struct Inner {
>      /// Array of pages.
>      ///
> @@ -308,6 +329,16 @@ pub(crate) fn register_with_vma(&self, vma: &virt::VmaNew) -> Result<usize> {
>          inner.size = num_pages;
>          inner.vma_addr = vma.start();
>  
> +        // This pointer is only used for comparison - it's not dereferenced.
> +        //
> +        // SAFETY: We own the vma, and we don't use any methods on VmaNew that rely on
> +        // `vm_private_data`.
> +        unsafe { (*vma.as_ptr()).vm_private_data = self as *const Self as *mut c_void };

Maybe use from_ref(self).cast_mut().cast::<c_void>() instead?

Please don't consider any of those NITs a blocker. :)