linux-kernel - Re: [PATCH v4 8/9] rust: file: add `DeferredFdCloser`

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Date: Mon, 05 Feb 2024 12:18:19 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...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>, Andreas Hindborg <a.hindborg@...sung.com>, Peter Zijlstra <peterz@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 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>, Kees Cook <keescook@...omium.org>, Matthew Wilcox <willy@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Cc: Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 8/9] rust: file: add `DeferredFdCloser`

On 2/2/24 11:55, Alice Ryhl wrote:
> To close an fd from kernel space, we could call `ksys_close`. However,
> if we do this to an fd that is held using `fdget`, then we may trigger a
> use-after-free. Introduce a helper that can be used to close an fd even
> if the fd is currently held with `fdget`. This is done by grabbing an
> extra refcount to the file and dropping it in a task work once we return
> to userspace.
> 
> This is necessary for Rust Binder because otherwise the user might try
> to have Binder close its fd for /dev/binder, which would cause problems
> as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
> using `fdget`.
> 
> Additional motivation can be found in commit 80cd795630d6 ("binder: fix
> use-after-free due to ksys_close() during fdget()") and in the comments
> on `binder_do_fd_close`.
> 
> If there is some way to detect whether an fd is currently held with
> `fdget`, then this could be optimized to skip the allocation and task
> work when this is not the case. Another possible optimization would be
> to combine several fds into a single task work, since this is used with
> fd arrays that might hold several fds.
> 
> That said, it might not be necessary to optimize it, because Rust Binder
> has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
> BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
> this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
> rarely these days.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/bindings/bindings_helper.h |   2 +
>  rust/helpers.c                  |   8 ++
>  rust/kernel/file.rs             | 184 +++++++++++++++++++++++++++++++-
>  rust/kernel/task.rs             |  14 +++
>  4 files changed, 207 insertions(+), 1 deletion(-)
> 

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

--
Cheers,
Benno