[PATCH v10 4/8] rust: cred: add Rust abstraction for `struct cred`

[Kees Cook]

On Sun, Sep 15, 2024 at 02:31:30PM +0000, Alice Ryhl wrote:
> From: Wedson Almeida Filho <wedsonaf at gmail.com>
> 
> Add a wrapper around `struct cred` called `Credential`, and provide
> functionality to get the `Credential` associated with a `File`.
> 
> Rust Binder must check the credentials of processes when they attempt to
> perform various operations, and these checks usually take a
> `&Credential` as parameter. The security_binder_set_context_mgr function
> would be one example. This patch is necessary to access these security_*
> methods from Rust.
> 
> This Rust abstraction makes the following assumptions about the C side:
> * `struct cred` is refcounted with `get_cred`/`put_cred`.

Yes

> * It's okay to transfer a `struct cred` across threads, that is, you do
>   not need to call `put_cred` on the same thread as where you called
>   `get_cred`.

Yes

> * The `euid` field of a `struct cred` never changes after
>   initialization.

"after initialization", yes. The bprm cred during exec is special in
that it gets updated (bprm_fill_uid) before it is installed into current
via commit_creds() in begin_new_exec() (the point of no return for
exec).

> * The `f_cred` field of a `struct file` never changes after
>   initialization.

Yes.

> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf at gmail.com>
> Co-developed-by: Alice Ryhl <aliceryhl at google.com>
> Reviewed-by: Trevor Gross <tmgross at umich.edu>
> Reviewed-by: Benno Lossin <benno.lossin at proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku at gmail.com>
> Reviewed-by: Gary Guo <gary at garyguo.net>
> Signed-off-by: Alice Ryhl <aliceryhl at google.com>

Reviewed-by: Kees Cook <kees at kernel.org>

-- 
Kees Cook