linux-kernel - Re: [PATCH v5 1/4] rust: uaccess: add userspace pointers

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Message-ID: <CAH5fLgjT3hAdtdNeb7FgX491UhvMGa-JHevz_EqC=N4zVViBjw@mail.gmail.com>
Date: Mon, 15 Apr 2024 11:44:33 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kees Cook <keescook@...omium.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v5 1/4] rust: uaccess: add userspace pointers

On Mon, Apr 15, 2024 at 11:37 AM Benno Lossin <benno.lossin@...ton.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 bytes.
> > +    ///
> > +    /// Constructing a [`UserSlice`] performs no checks on the provided address and length, it can
> > +    /// safely be constructed inside a kernel thread with no current userspace 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 within the user range (no
> > +    /// additional calls to `access_ok` are needed).
> > +    ///
> > +    /// Callers must be careful to avoid time-of-check-time-of-use (TOCTOU) issues. The simplest way
> > +    /// is to create a single instance of [`UserSlice`] per user memory 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 = [MaybeUninit::uninit(); 64];
>      let ptr: *mut [MaybeUninit<u8>] = &mut arr;
>      let ptr = ptr.cast::<c_void>();
>
>      let slice = UserSlice::new(ptr, 64);
>      let (mut r, mut w) = 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