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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 15 Apr 2024 11:44:33 +0200
Message-ID: <CAH5fLgjT3hAdtdNeb7FgX491UhvMGa-JHevz_EqC=N4zVViBjw@mail.gmail.com>
Subject: Re: [PATCH v5 1/4] rust: uaccess: add userspace pointers
To: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Matthew Wilcox <willy@infradead.org>, 
	Al Viro <viro@zeniv.linux.org.uk>, Andrew Morton <akpm@linux-foundation.org>, 
	Kees Cook <keescook@chromium.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	=?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, 
	Christian Brauner <brauner@kernel.org>
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On Mon, Apr 15, 2024 at 11:37=E2=80=AFAM Benno Lossin <benno.lossin@proton.=
me> wrote:
>
> On 15.04.24 09:13, Alice Ryhl wrote:
> > +impl UserSlice {
> > +    /// Constructs a user slice from a raw pointer and a length in byt=
es.
> > +    ///
> > +    /// Constructing a [`UserSlice`] performs no checks on the provide=
d address and length, it can
> > +    /// safely be constructed inside a kernel thread with no current u=
serspace process. Reads and
> > +    /// writes wrap the kernel APIs `copy_from_user` and `copy_to_user=
`, which check the memory map
> > +    /// of the current process and enforce that the address range is w=
ithin the user range (no
> > +    /// additional calls to `access_ok` are needed).
> > +    ///
> > +    /// Callers must be careful to avoid time-of-check-time-of-use (TO=
CTOU) issues. The simplest way
> > +    /// is to create a single instance of [`UserSlice`] per user memor=
y block as it reads each byte
> > +    /// at most once.
> > +    pub fn new(ptr: *mut c_void, length: usize) -> Self {
>
> What would happen if I call this with a kernel pointer and then
> read/write to it? For example
>
>      let mut arr =3D [MaybeUninit::uninit(); 64];
>      let ptr: *mut [MaybeUninit<u8>] =3D &mut arr;
>      let ptr =3D ptr.cast::<c_void>();
>
>      let slice =3D UserSlice::new(ptr, 64);
>      let (mut r, mut w) =3D slice.reader_writer();
>
>      r.read_raw(&mut arr)?;
>      // SAFETY: `arr` was initialized above.
>      w.write_slice(unsafe { MaybeUninit::slice_assume_init_ref(&arr) })?;
>
> I think this would violate the exclusivity of `&mut` without any
> `unsafe` code. (the `unsafe` block at the end cannot possibly be wrong)

This will fail with an EFAULT error. There is a check on the C side
that verifies that the address is in userspace. (The access_ok call.)

Alice