Re: [PATCH v4 08/13] rust: introduce `ARef`

[Benno Lossin <benno.lossin@proton.me>]

On 11.04.23 07:45, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
>
> This is an owned reference to an object that is always ref-counted. This
> is meant to be used in wrappers for C types that have their own ref
> counting functions, for example, tasks, files, inodes, dentries, etc.
>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> ---
> v1 -> v2: No changes
> v2 -> v3: No changes
> v3 -> v4: No changes
>
>   rust/kernel/types.rs | 107 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 107 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index a4b1e3778da7..29db59d6119a 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -6,8 +6,10 @@ use crate::init::{self, PinInit};
>   use alloc::boxed::Box;
>   use core::{
>       cell::UnsafeCell,
> +    marker::PhantomData,
>       mem::MaybeUninit,
>       ops::{Deref, DerefMut},
> +    ptr::NonNull,
>   };
>
>   /// Used to transfer ownership to and from foreign (non-Rust) languages.
> @@ -268,6 +270,111 @@ impl<T> Opaque<T> {
>       }
>   }
>
> +/// Types that are _always_ reference counted.
> +///
> +/// It allows such types to define their own custom ref increment and decrement functions.
> +/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
> +/// [`ARef<T>`].
> +///
> +/// This is usually implemented by wrappers to existing structures on the C side of the code. For
> +/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
> +/// instances of a type.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that increments to the reference count keep the object alive in memory
> +/// at least until matching decrements are performed.
> +///
> +/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
> +/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
> +/// alive.)

`dec_ref` states below that it 'Frees the object when the count reaches
zero.', this should also be stated here, since implementers should adhere
to that when implementing `dec_ref`.

> +pub unsafe trait AlwaysRefCounted {
> +    /// Increments the reference count on the object.
> +    fn inc_ref(&self);



> +
> +    /// Decrements the reference count on the object.
> +    ///
> +    /// Frees the object when the count reaches zero.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that there was a previous matching increment to the reference count,
> +    /// and that the object is no longer used after its reference count is decremented (as it may
> +    /// result in the object being freed), unless the caller owns another increment on the refcount
> +    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
> +    /// [`AlwaysRefCounted::dec_ref`] once).
> +    unsafe fn dec_ref(obj: NonNull<Self>);
> +}
> +
> +/// An owned reference to an always-reference-counted object.
> +///
> +/// The object's reference count is automatically decremented when an instance of [`ARef`] is
> +/// dropped. It is also automatically incremented when a new instance is created via
> +/// [`ARef::clone`].
> +///
> +/// # Invariants
> +///
> +/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
> +/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
> +pub struct ARef<T: AlwaysRefCounted> {
> +    ptr: NonNull<T>,
> +    _p: PhantomData<T>,
> +}
> +
> +impl<T: AlwaysRefCounted> ARef<T> {
> +    /// Creates a new instance of [`ARef`].
> +    ///
> +    /// It takes over an increment of the reference count on the underlying object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that the reference count was incremented at least once, and that they
> +    /// are properly relinquishing one increment. That is, if there is only one increment, callers
> +    /// must not use the underlying object anymore -- it is only safe to do so via the newly
> +    /// created [`ARef`].
> +    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
> +        // INVARIANT: The safety requirements guarantee that the new instance now owns the
> +        // increment on the refcount.
> +        Self {
> +            ptr,
> +            _p: PhantomData,
> +        }
> +    }
> +}
> +
> +impl<T: AlwaysRefCounted> Clone for ARef<T> {
> +    fn clone(&self) -> Self {
> +        self.inc_ref();
> +        // SAFETY: We just incremented the refcount above.
> +        unsafe { Self::from_raw(self.ptr) }
> +    }
> +}
> +
> +impl<T: AlwaysRefCounted> Deref for ARef<T> {
> +    type Target = T;
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: The type invariants guarantee that the object is valid.
> +        unsafe { self.ptr.as_ref() }
> +    }
> +}
> +
> +impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
> +    fn from(b: &T) -> Self {
> +        b.inc_ref();
> +        // SAFETY: We just incremented the refcount above.
> +        unsafe { Self::from_raw(NonNull::from(b)) }
> +    }
> +}

This impl seems unsound to me, as we can do this:

     struct MyStruct {
         raw: Opaque<bindings::my_struct>, // This has a `refcount_t` inside.
     }

     impl MyStruct {
         fn new() -> Self { ... }
     }

     unsafe impl AlwaysRefCounted for MyStruct { ... } // Implemented correctly.

     fn evil() -> ARef<MyStruct> {
         let my_struct = MyStruct::new();
         ARef::from(&my_struct) // We return a pointer to the stack!
     }

similarly, this can also be done with a `Box`:

     fn evil2() -> ARef<MyStruct> {
         let my_struct = Box::new(MyStruct::new());
         ARef::from(&*my_struct)
         // Box is freed here, even just dropping the `ARef` will result in
         // a UAF.
     }

Additionally, I think that `AlwaysRefCounted::inc_ref` should not be safe,
as the caller must not deallocate the memory until the refcount is zero.

Another pitfall of `ARef`: it does not deallocate the memory when the
refcount reaches zero. People might expect that this code would not leak
memory:

     let foo = Box::try_new(Foo::new())?;
     let foo = Box::leak(foo); // Leak the box, such that we do not
                               // deallocate the memory too early.
     let foo = ARef::from(foo);
     drop(foo); // refcount is now zero, but the memory is never deallocated.


> +
> +impl<T: AlwaysRefCounted> Drop for ARef<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
> +        // decrement.
> +        unsafe { T::dec_ref(self.ptr) };
> +    }
> +}
> +
>   /// A sum type that always holds either a value of type `L` or `R`.
>   pub enum Either<L, R> {
>       /// Constructs an instance of [`Either`] containing a value of type `L`.
> --
> 2.34.1
>