linux-kernel - Re: [PATCH v2] x86,ibt: Use UDB instead of 0xEA

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Message-ID: <20250902195735.GM3245006@noisy.programming.kicks-ass.net>
Date: Tue, 2 Sep 2025 21:57:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <kees@...nel.org>, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, Joao Moreira <joao@...rdrivepizza.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>, ojeda@...nel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86,ibt: Use UDB instead of 0xEA

On Tue, Sep 02, 2025 at 12:37:26PM -0700, Alexei Starovoitov wrote:

> Well, I mean all these tricky changes are allegedly because
> "Intel/AMD have not blessed using this instruction, it is on
> their 'reserved' opcode list for future use"
> 
> I suspect that 'reserved' opcode will not be used any time soon.
> If 10 years from now the opcode is used in some future CPU that CPU
> is better to be not vulnerable and CFI, FineIBT things will be
> gone from the kernel by then.
> So I would do absolutely nothing and just ignore the lack of blessing.

Ah. CFI is not only about speculation, it is also very much a protection
against pointer hijacking. Note how kCFI was not brought in because of
speculation, but as a hardening against pointer hijacking.

Yes, my interest in Intel CET-IBT is mostly from the speculation
avoidance angle, but CFI as a whole has wider applicability. Even a
'fixed' CPU would want to use CFI.

I would also love AMD to grow support for CET-IBT; they already
implemented CET-SS.

Also, Intel pays me, if they say make it use 0xD6, this is what we do
:-)