Re: [PATCH v2] uaccess: rust: use newtype for user pointers

[Boqun Feng]

On Tue, May 27, 2025 at 11:12:11PM +0100, Al Viro wrote:
> On Tue, May 27, 2025 at 01:53:12PM +0000, Alice Ryhl wrote:
> > In C code we use sparse with the __user annotation to detect cases where
> > a user pointer is mixed up with other things. To replicate that, we
> > introduce a new struct UserPtr that serves the same purpose using the
> > newtype pattern.
> > 
> > The UserPtr type is not marked with #[derive(Debug)], which means that
> > it's not possible to print values of this type. This avoids ASLR
> > leakage.
> > 
> > The type is added to the prelude as it is a fairly fundamental type
> > similar to c_int. The wrapping_add() method is renamed to
> > wrapping_byte_add() for consistency with the method name found on raw
> > pointers.
> 
> That's considerably weaker than __user, though - with
> 	struct foo {struct bar x; struct baz y[2]; };

Translate to Rust this is:

    struct Foo {
        x: Bar,
	y: Baz[2],
    }

> 	struct foo __user *p;

UserPtr should probably be generic over pointee, so:

    pub struct UserPtr<T>(*mut c_void, PhantomData<*mut T>);

and

    let p: UserPtr<Foo> = ...;

> 	void f(struct bar __user *);

and this is:

    pub fn f(bar: UserPtr<Bar>)

and the checking should work, a (maybe unrelated) tricky part though..

> sparse does figure out that f(&p->y[1]) is a type error - &p->y[1] is

In Rust, you will need to play a little unsafe game to get &p->y[1]:

    let foo_ptr: *mut Foo = p.as_mut_ptr();
    let y_ptr: *mut Baz = unsafe { addr_of_mut!((*foo_ptr).y[1]) };
    let y: UserPtr<Baz> = unsafe { UserPtr::from_ptr(y_ptr) };

passing y to f() will get a type mismatch, so the detection/checking
works. To avoid the unsafe game we need field projection [1].

> struct baz __user * and f() expects struct bar __user *.
> 
> It's not just mixing userland pointers with other things - it's not mixing
> userland pointers to different types, etc.
> 

In short, with UserPtr generic over pointee, we can have the similar
detection as sparse.

[1]: https://github.com/rust-lang/rfcs/pull/3735

Regards,
Boqun

> In practice I've seen quite a few brainos caught by that...