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

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Apr 17, 2024 at 4:28 PM Gary Guo <gary@garyguo.net> wrote:
>
> On Mon, 15 Apr 2024 07:13:53 +0000
> Alice Ryhl <aliceryhl@google.com> 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>
> > ---
> >  rust/helpers.c         |  14 +++
> >  rust/kernel/lib.rs     |   1 +
> >  rust/kernel/uaccess.rs | 304 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 319 insertions(+)
> >
> > diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
>
> > +/// [`std::io`]: https://doc.rust-lang.org/std/io/index.html
> > +/// [`clone_reader`]: UserSliceReader::clone_reader
> > +pub struct UserSlice {
> > +    ptr: *mut c_void,
> > +    length: usize,
> > +}
>
> How useful is the `c_void` in the struct and new signature? They tend
> to not be very useful in Rust. Given that provenance doesn't matter
> for userspace pointers, could this be `usize` simply?
>
> I think `*mut u8` or `*mut ()` makes more sense than `*mut c_void` for
> Rust code even if we don't want to use `usize`.

I don't have a strong opinion here. I suppose a usize could make
sense. But I also think c_void is fine, and I lean towards not
changing it. :)

> Some thinking aloud and brainstorming bits about the API.
>
> I wonder if it make sense to have `User<[u8]>` instead of `UserSlice`?
> The `User` type can be defined like this:
>
> ```rust
> struct User<T: ?Sized> {
>    ptr: *mut T,
> }
> ```
>
> and this allows arbitrary T as long as it's POD. So we could have
> `User<[u8]>`, `User<u32>`, `User<PodStruct>`. I imagine the
> `User<[u8]>` would be the general usage and the latter ones can be
> especially helpful if you are trying to implement ioctl and need to
> copy fixed size data structs from userspace.

Hmm, we have to be careful here. Generally, when you get a user slice
via an ioctl, you should make sure to use the length you get from
userspace. In binder, it looks like this:

let user_slice = UserSlice::new(arg, _IOC_SIZE(cmd));

so whichever API we use, we must make sure to get the length as an
argument in bytes. What should we do if the length is not a multiple
of size_of(T)?

Another issue is that there's no stable way to get the length from a
`*mut [T]` without creating a reference, which is not okay for a user
slice.

Alice