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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Apr 15, 2024 at 07:13:53AM +0000, Alice Ryhl wrote:
> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> 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@gmail.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Thanks!

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Two small nits below..

> ---
>  rust/helpers.c         |  14 +++
>  rust/kernel/lib.rs     |   1 +
>  rust/kernel/uaccess.rs | 304 +++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 319 insertions(+)
> 
[...]
> +    /// Reads raw data from the user slice into a kernel buffer.
> +    ///
> +    /// Fails with `EFAULT` if the read happens on a bad address.

... we probably want to mention that `out` may get modified even in
failure cases.

> +    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)
> +    }
> +
[...]
> +
> +impl UserSliceWriter {
[...]
> +
> +    /// Writes raw data to this user pointer from a kernel buffer.
> +    ///
> +    /// Fails with `EFAULT` if the write happens on a bad address.

Same here, probably mention that: the userspace memory may be modified
even in failure cases.

Anyway, they are not correctness critical, so we can do these in later
patches.

Regards,
Boqun

> +    pub fn write_slice(&mut self, data: &[u8]) -> Result {
> +        let len = data.len();
> +        let data_ptr = data.as_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: `data_ptr` points into an immutable slice of length `len_ulong`, so we may read
> +        // that many bytes from it.
> +        let res = unsafe { bindings::copy_to_user(self.ptr, data_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(())
> +    }
> +}
> 
> -- 
> 2.44.0.683.g7961c838ac-goog
> 
>