linux-kernel - Re: [PATCH] x86/Kconfig: make CFI_AUTO

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Message-ID: <CAH5fLggMoo8SNetgm1cC5u6ZabbhA5o8L3U2x5p4NP2n37F7xw@mail.gmail.com>
Date: Thu, 10 Apr 2025 15:20:49 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Paweł Anikiel <panikiel@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Kees Cook <kees@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Nathan Chancellor <nathan@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] x86/Kconfig: make CFI_AUTO_DEFAULT depend on !RUST

On Thu, Apr 10, 2025 at 3:09 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Apr 10, 2025 at 02:45:26PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 10, 2025 at 02:36:02PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 10, 2025 at 11:54:20AM +0000, Paweł Anikiel wrote:
> > > > Calling core::fmt::write() from rust code while FineIBT is enabled
> > > > results in a kernel panic:
>
> > > > This happens because core::fmt::write() calls
> > > > core::fmt::rt::Argument::fmt(), which currently has CFI disabled:
> > > >
> > > > library/core/src/fmt/rt.rs:
> > > > 171     // FIXME: Transmuting formatter in new and indirectly branching to/calling
> > > > 172     // it here is an explicit CFI violation.
> > > > 173     #[allow(inline_no_sanitize)]
> > > > 174     #[no_sanitize(cfi, kcfi)]
> > > > 175     #[inline]
> > > > 176     pub(super) unsafe fn fmt(&self, f: &mut Formatter<'_>) -> Result {
> > > >
>
> Miguel, I cannot find this code in the kernel tree. Is this again
> because Rust is not free-standing and relies on external code?
>
> Can you please fix that. Building against external code that is not
> under our control is a problem.

It's from the core standard library, which is shipped together with
the compiler.

Alice