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

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Message-ID: <CAH5fLgjRpY9vYWw0T0g3R_zndvj6AGKHeHw=H2yM+C5-SHt6BQ@mail.gmail.com>
Date: Sun, 15 Sep 2024 22:55:14 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Kees Cook <kees@...nel.org>
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 10:24 PM Kees Cook <kees@...nel.org> wrote:
>
> 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).

I think it will be pretty normal to need different Rust types for pre-
and post-initialization of a C value. When a value is not yet fully
initialized, you usually get some extra powers (modify otherwise
immutable fields), but probably also lose some powers (you can't share
it with other threads yet). I can document that this type should not
be used with the bprm cred during 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>

Thanks for the review!

Alice