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

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Message-ID: <202409151318.7985B253@keescook>
Date: Sun, 15 Sep 2024 13:24:38 -0700
From: Kees Cook <kees@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Carlos Llamas <cmllamas@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, Daniel Xu <dxu@...uu.xyz>,
	Martin Rodriguez Reboredo <yakoyoku@...il.com>,
	Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v10 4/8] rust: cred: add Rust abstraction for `struct
 cred`

On Sun, Sep 15, 2024 at 02:31:30PM +0000, Alice Ryhl wrote:
> From: Wedson Almeida Filho <wedsonaf@...il.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@...il.com>
> Co-developed-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Trevor Gross <tmgross@...ch.edu>
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

Reviewed-by: Kees Cook <kees@...nel.org>

-- 
Kees Cook