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

[Trevor Gross]

On Mon, Apr 15, 2024 at 3:14 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
>
> A pointer to an area in userspace memory, which can be either read-only
> or read-write.
>
> All methods on this struct are safe: attempting to read or write on bad
> addresses (either out of the bound of the slice or unmapped addresses)
> will return `EFAULT`. Concurrent access, *including data races to/from
> userspace memory*, is permitted, because fundamentally another userspace
> thread/process could always be modifying memory at the same time (in the
> same way that userspace Rust's `std::io` permits data races with the
> contents of files on disk). In the presence of a race, the exact byte
> values read/written are unspecified but the operation is well-defined.
> Kernelspace code should validate its copy of data after completing a
> read, and not expect that multiple reads of the same address will return
> the same value.
>
> These APIs are designed to make it difficult to accidentally write
> TOCTOU bugs. Every time you read from a memory location, the pointer is
> advanced by the length so that you cannot use that reader to read the
> same memory location twice. Preventing double-fetches avoids TOCTOU
> bugs. This is accomplished by taking `self` by value to prevent
> obtaining multiple readers on a given `UserSlicePtr`, and the readers
> only permitting forward reads. If double-fetching a memory location is
> necessary for some reason, then that is done by creating multiple
> readers to the same memory location.
>
> Constructing a `UserSlicePtr` 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).
>
> This code is based on something that was originally written by Wedson on
> the old rust branch. It was modified by Alice by removing the
> `IoBufferReader` and `IoBufferWriter` traits, and various other changes.
>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> Co-developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>

I left some suggestions for documentation improvements and one
question, but mostly LGTM.

> ---
>  rust/helpers.c         |  14 +++
>  rust/kernel/lib.rs     |   1 +
>  rust/kernel/uaccess.rs | 304 +++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 319 insertions(+)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index be68d5e567b1..37f84223b83f 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -49,6 +49,7 @@
>  pub mod task;
>  pub mod time;
>  pub mod types;
> +pub mod uaccess;
>  pub mod workqueue;
>
>  #[doc(hidden)]
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> new file mode 100644
> index 000000000000..c97029cdeba1
> --- /dev/null
> +++ b/rust/kernel/uaccess.rs
> @@ -0,0 +1,304 @@
> [...]
> +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).

I would just add a note that pointer should be a valid userspace
pointer, but that gets checked at read/write time

> +    /// 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 {
> +        UserSlice { ptr, length }
> +    }

> +impl UserSliceReader {
> [...]
> +    /// Reads raw data from the user slice into a kernel buffer.
> +    ///
> +    /// After a successful call to this method, all bytes in `out` are initialized.

If this is guaranteed, could it return `Result<&mut [u8]>`? So the
caller doesn't need to unsafely `assume_init` anything.

> +    /// Fails with `EFAULT` if the read happens on a bad address.

This should also mention that the slice cannot be bigger than the
reader's length.

> +    pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
> +        let len = out.len();
> +        let out_ptr = out.as_mut_ptr().cast::<c_void>();
> +        if len > self.length {
> +            return Err(EFAULT);
> +        }
> +        let Ok(len_ulong) = c_ulong::try_from(len) else {
> +            return Err(EFAULT);
> +        };
> +        // SAFETY: `out_ptr` points into a mutable slice of length `len_ulong`, so we may write
> +        // that many bytes to it.
> +        let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr, len_ulong) };
> +        if res != 0 {
> +            return Err(EFAULT);
> +        }
> +        // Userspace pointers are not directly dereferencable by the kernel, so we cannot use `add`,
> +        // which has C-style rules for defined behavior.
> +        self.ptr = self.ptr.wrapping_byte_add(len);
> +        self.length -= len;
> +        Ok(())
> +    }
> +
> +    /// Reads raw data from the user slice into a kernel buffer.
> +    ///
> +    /// Fails with `EFAULT` if the read happens on a bad address.
> +    pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
> +        // SAFETY: The types are compatible and `read_raw` doesn't write uninitialized bytes to
> +        // `out`.
> +        let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
> +        self.read_raw(out)
> +    }

If this is just a safe version of read_raw, could you crosslink the docs?

> +impl UserSliceWriter {
> +
> +    /// Writes raw data to this user pointer from a kernel buffer.
> +    ///
> +    /// Fails with `EFAULT` if the write happens on a bad address.
> +    pub fn write_slice(&mut self, data: &[u8]) -> Result {
>   [...]
> +    }

Could use a note about length like `read_raw`.