Re: [PATCH v5 3/4] rust: uaccess: add typed accessors for userspace pointers

[Trevor Gross <tmgross@umich.edu>]

On Mon, Apr 15, 2024 at 3:15 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> Add safe methods for reading and writing Rust values to and from
> userspace pointers.
>
> The C methods for copying to/from userspace use a function called
> `check_object_size` to verify that the kernel pointer is not dangling.
> However, this check is skipped when the length is a compile-time
> constant, with the assumption that such cases trivially have a correct
> kernel pointer.
>
> In this patch, we apply the same optimization to the typed accessors.
> For both methods, the size of the operation is known at compile time to
> be size_of of the type being read or written. Since the C side doesn't
> provide a variant that skips only this check, we create custom helpers
> for this purpose.
>
> The majority of reads and writes to userspace pointers in the Rust
> Binder driver uses these accessor methods. Benchmarking has found that
> skipping the `check_object_size` check makes a big difference for the
> cases being skipped here. (And that the check doesn't make a difference
> for the cases that use the raw read/write methods.)
>
> This code is based on something that was originally written by Wedson on
> the old rust branch. It was modified by Alice to skip the
> `check_object_size` check, and to update various comments, including the
> notes about kernel pointers in `WritableToBytes`.
>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Couple of docs nits but this looks good to me.

Reviewed-by: Trevor Gross <tmgross@umich.edu>

> +/// Types for which any bit pattern is valid.
> +///
> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> +/// reading arbitrary bytes into something that contains a `bool` is not okay.
> +///
> +/// It's okay for the type to have padding, as initializing those bytes has no effect.
> +///
> +/// # Safety
> +///
> +/// All bit-patterns must be valid for this type.
> +pub unsafe trait FromBytes {}

No `UnsafeCell` is also a requirement in zerocopy/bytemuck

> +/// Types that can be viewed as an immutable slice of initialized bytes.
> +///
> +/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This
> +/// means that it should not have any padding, as padding bytes are uninitialized. Reading
> +/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive
> +/// information on the stack to userspace.
> +///
> +/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered
> +/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so
> +/// this is a correctness requirement, but not a safety requirement.

I don't think mentions of userspace are relevant here since the trait
is more general. Maybe a `# Interfacing with userspace` section if
there is enough relevant information.

> +/// # Safety
> +///
> +/// Values of this type may not contain any uninitialized bytes.

No UnsafeCell

> +pub unsafe trait AsBytes {}

> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index c97029cdeba1..e3953eec61a3 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -4,10 +4,15 @@
>  //!
>  //! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h)
>
> -use crate::{bindings, error::code::*, error::Result};
> +use crate::{
> +    bindings,
> +    error::code::*,
> +    error::Result,
> +    types::{AsBytes, FromBytes},
> +};
>  use alloc::vec::Vec;
>  use core::ffi::{c_ulong, c_void};
> -use core::mem::MaybeUninit;
> +use core::mem::{size_of, MaybeUninit};
>
>  /// A pointer to an area in userspace memory, which can be either read-only or read-write.
>  ///
> @@ -238,6 +243,38 @@ pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
>          self.read_raw(out)
>      }
>
> +    /// Reads a value of the specified type.
> +    ///
> +    /// Fails with `EFAULT` if the read encounters a page fault.
> +    pub fn read<T: FromBytes>(&mut self) -> Result<T> {
> [...]
> +    /// Writes the provided Rust value to this userspace pointer.
> +    ///
> +    /// Fails with `EFAULT` if the write encounters a page fault.
> +    pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {

Read & write could use an example if you are up for it