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From:   Ramon de C Valle <rcvalle@google.com>
Date:   Thu, 12 Oct 2023 15:26:51 -0700
Message-ID: <CAOcBZOQ=M7zpXTA7Ue1=2436N4=MFgKVRPmTQ1H=b1mUwJEqEQ@mail.gmail.com>
Subject: Re: [PATCH v10 25/27] x86: enable initial Rust support
To:     Kees Cook <keescook@chromium.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, patches@lists.linux.dev,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        David Gow <davidgow@google.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-doc@vger.kernel.org
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On Thu, Oct 12, 2023 at 11:31=E2=80=AFAM Kees Cook <keescook@chromium.org> =
wrote:
>
> On Thu, Oct 12, 2023 at 10:50:36AM -0700, Sami Tolvanen wrote:
> > On Thu, Oct 12, 2023 at 3:47=E2=80=AFAM Peter Zijlstra <peterz@infradea=
d.org> wrote:
> > >
> > > On Fri, Oct 14, 2022 at 11:34:30AM -0700, Sami Tolvanen wrote:
> > > > On Fri, Oct 14, 2022 at 11:05 AM Miguel Ojeda
> > > > <miguel.ojeda.sandonis@gmail.com> wrote:
> > > > >
> > > > > On Tue, Oct 11, 2022 at 1:16 AM Sami Tolvanen <samitolvanen@googl=
e.com> wrote:
> > > > > >
> > > > > > Rust supports IBT with -Z cf-protection=3Dbranch, but I don't s=
ee this
> > > > > > option being enabled in the kernel yet. Cross-language CFI is g=
oing to
> > > > > > require a lot more work though because the type systems are not=
 quite
> > > > > > compatible:
> > > > > >
> > > > > > https://github.com/rust-lang/rfcs/pull/3296
> > > > >
> > > > > I have pinged Ramon de C Valle as he is the author of the RFC abo=
ve
> > > > > and implementation work too; since a month or so ago he also lead=
s the
> > > > > Exploit Mitigations Project Group in Rust.
> > > >
> > > > Thanks, Miguel. I also talked to Ramon about KCFI earlier this week
> > > > and he expressed interest in helping with rustc support for it. In =
the
> > > > meanwhile, I think we can just add a depends on !CFI_CLANG to avoid
> > > > issues here.
> > >
> > > Having just read up on the thing it looks like the KCFI thing is
> > > resolved.
> > >
> > > I'm not sure I understand most of the objections in that thread throu=
gh
> > > -- enabling CFI *will* break stuff, so what.
> > >
> > > Squashing the integer types seems a workable compromise I suppose. On=
e
> > > thing that's been floated in the past is adding a 'seed' attribute to
> > > some functions in order to distinguish functions of otherwise identic=
al
> > > signature.
> > >
> > > The Rust thing would then also need to support this attribute.
> > >
> > > Are there any concrete plans for this? It would allow, for example,
> > > to differentiate address_space_operations::swap_deactivate() from any
> > > other random function that takes only a file argument, say:
> > > locks_remove_file().
> >
> > I haven't really had time to look into it, so no concrete plans yet.
> > Adding an attribute shouldn't be terribly difficult, but Kees
> > expressed interest in automatic salting as well, which might be a more
> > involved project:
> >
> > https://github.com/ClangBuiltLinux/linux/issues/1736
>
> Automatic would be nice, but having an attribute would let us at least
> start the process manually (or apply salting from static analysis
> output, etc).

An idea would be to add something like the Rust cfi_encoding
attribute[1] and use it with something similar to the Newtype
Pattern[2], but in C[3], for aggregating function pointers that
otherwise would be aggregated in the same group in different groups.

[1]: https://doc.rust-lang.org/nightly/unstable-book/language-features/cfi-=
encoding.html
[2]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#using-the-n=
ewtype-pattern-for-type-safety-and-abstraction
[3]: Wrapping a type in a struct should achieve something similar even
without using the cfi_encoding attribute since the encoding for
structs is <length><name>, where <name> is <unscoped-name>.