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

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Message-ID: <20250418094521.GQ38216@noisy.programming.kicks-ass.net>
Date: Fri, 18 Apr 2025 11:45:21 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Kees Cook <kees@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
	Paweł Anikiel <panikiel@...gle.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	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,
	Matthew Maurer <mmaurer@...gle.com>,
	Ramon de C Valle <rcvalle@...gle.com>
Subject: Re: [PATCH] x86/Kconfig: make CFI_AUTO_DEFAULT depend on !RUST

On Thu, Apr 17, 2025 at 08:40:24PM +0200, Miguel Ojeda wrote:

> From 2c6636c82f8c6c18e18c085fe69d0cd3dbe3b991 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Pawe=C5=82=20Anikiel?= <panikiel@...gle.com>
> Date: Thu, 10 Apr 2025 11:54:20 +0000
> Subject: [PATCH] x86/Kconfig: make CFI_AUTO_DEFAULT depend on !RUST or Rust >=
>  1.88
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Calling core::fmt::write() from rust code while FineIBT is enabled
> results in a kernel panic:
> 
> [ 4614.199779] kernel BUG at arch/x86/kernel/cet.c:132!
> [ 4614.205343] Oops: invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> [ 4614.211781] CPU: 2 UID: 0 PID: 6057 Comm: dmabuf_dump Tainted: G     U     O       6.12.17-android16-0-g6ab38c534a43 #1 9da040f27673ec3945e23b998a0f8bd64c846599
> [ 4614.227832] Tainted: [U]=USER, [O]=OOT_MODULE
> [ 4614.241247] RIP: 0010:do_kernel_cp_fault+0xea/0xf0

Might want to 'compress' the splat a bit, lots of useless bits on etc.

> [ 4614.398144] RIP: 0010:_RNvXs5_NtNtNtCs3o2tGsuHyou_4core3fmt3num3impyNtB9_7Display3fmt+0x0/0x20
> [ 4614.407792] Code: 48 f7 df 48 0f 48 f9 48 89 f2 89 c6 5d e9 18 fd ff ff 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 41 81 ea 14 61 af 2c 74 03 0f 0b 90 <66> 0f 1f 00 55 48 89 e5 48 89 f2 48 8b 3f be 01 00 00 00 5d e9 e7
> [ 4614.428775] RSP: 0018:ffffb95acfa4ba68 EFLAGS: 00010246
> [ 4614.434609] RAX: 0000000000000000 RBX: 0000000000000010 RCX: 0000000000000000
> [ 4614.442587] RDX: 0000000000000007 RSI: ffffb95acfa4ba70 RDI: ffffb95acfa4bc88
> [ 4614.450557] RBP: ffffb95acfa4bae0 R08: ffff0a00ffffff05 R09: 0000000000000070
> [ 4614.458527] R10: 0000000000000000 R11: ffffffffab67eaf0 R12: ffffb95acfa4bcc8
> [ 4614.466493] R13: ffffffffac5d50f0 R14: 0000000000000000 R15: 0000000000000000
> [ 4614.474473]  ? __cfi__RNvXs5_NtNtNtCs3o2tGsuHyou_4core3fmt3num3impyNtB9_7Display3fmt+0x10/0x10
> [ 4614.484118]  ? _RNvNtCs3o2tGsuHyou_4core3fmt5write+0x1d2/0x250
> 
> 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 {
> 
> This causes a Control Protection exception, because FineIBT has sealed
> off the original function's endbr64.

Right, doing an indirect call without the CFI adornment to a CFI adorned
function is explicitly not supported.

> This makes rust currently incompatible with FineIBT. Add a Kconfig
> dependency that prevents FineIBT from getting turned on by default
> if rust is enabled.
> 
> [ Rust 1.88.0 (scheduled for 2025-06-26) should have this fixed [1],
>   and thus we relaxed the condition with Rust >= 1.88.
> 
>   When `objtool` lands checking for this with e.g. [2], the plan is
>   to ideally run that in upstream Rust's CI to prevent regressions
>   early [3], since we do not control `core`'s source code.

I'm still firmly of the opinion Rust should grow up and have a stable
core which we can pull into the kernel. Relying on that much out of tree
code is 'unfortunate'.

>   Alice tested the Rust PR backported to an older compiler.
> 
>     - Miguel ]
> 
> Signed-off-by: Paweł Anikiel <panikiel@...gle.com>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Acked-by: Peter Zijlstra <peterz@...radead.org>
> Link: https://github.com/rust-lang/rust/pull/139632 [1]
> Link: https://lore.kernel.org/rust-for-linux/20250410154556.GB9003@noisy.programming.kicks-ass.net/ [2]
> Link: https://github.com/rust-lang/rust/pull/139632#issuecomment-2801950873 [3]
> Link: https://lore.kernel.org/r/20250410115420.366349-1-panikiel@google.com
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> ---
>  arch/x86/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4b9f378e05f6..5873c9e39919 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2368,6 +2368,7 @@ config STRICT_SIGALTSTACK_SIZE
>  config CFI_AUTO_DEFAULT
>  	bool "Attempt to use FineIBT by default at boot time"
>  	depends on FINEIBT
> +	depends on !RUST || RUSTC_VERSION >= 108800
>  	default y
>  	help
>  	  Attempt to use FineIBT by default at boot time. If enabled,
> 
> base-commit: c1b4071ec3a6a594df6c49bf8f04a60a88072525
> -- 
> 2.49.0
>