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From: Alice Ryhl <aliceryhl@google.com>
Date: Wed, 14 Feb 2024 11:51:36 +0100
Message-ID: <CAH5fLggZc_Fxi1gjx9jxGDgyYOj1NLz5MYxzRxEja6vV0WffGg@mail.gmail.com>
Subject: Re: [PATCH v2 2/4] uaccess: always export _copy_[from|to]_user with CONFIG_RUST
To: Kees Cook <keescook@chromium.org>
Cc: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Al Viro <viro@zeniv.linux.org.uk>, Andrew Morton <akpm@linux-foundation.org>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, 
	Christian Brauner <brauner@kernel.org>
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On Sat, Feb 10, 2024 at 1:15=E2=80=AFAM Kees Cook <keescook@chromium.org> w=
rote:
>
> On Thu, Feb 08, 2024 at 03:47:52PM +0000, Alice Ryhl wrote:
> >       unsigned long res =3D n;
> >       might_fault();
> >       if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> > +             /*
> > +              * Ensure that bad access_ok() speculation will not
> > +              * lead to nasty side effects *after* the copy is
> > +              * finished:
> > +              */
> > +             barrier_nospec();
>
> This means all callers just gained this barrier. That's a behavioral
> change -- is it intentional here? I don't see it mentioned in the commit
> log.
>
> Also did this get tested with the CONFIG_TEST_USER_COPY tests? I would
> expect it to be fine, but better to check and mention it in the commit
> log.

I just ran this with CONFIG_TEST_USER_COPY on x86 using the Android
cuttlefish emulator and it passed there. I also verified that it fails
if I remove the access_ok check. However, the tests succeed even if
the barrier_nospec() call is removed.

That said, it seems like it fails to compile on some other platforms.
It seems like we need to add #include <linux/nospec.h> to uaccess.h to
fix it.

Alice