linux-kernel - Re: [PATCH] x86/kcfi: Require FRED for FineIBT

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Message-ID: <202502161552.54EA17D@keescook>
Date: Sun, 16 Feb 2025 15:52:40 -0800
From: Kees Cook <kees@...nel.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Jennifer Miller <jmill@....edu>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Jann Horn <jannh@...gle.com>, Nathan Chancellor <nathan@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>,
	Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/kcfi: Require FRED for FineIBT

On Fri, Feb 14, 2025 at 10:40:28PM +0000, Andrew Cooper wrote:
> On 14/02/2025 9:54 pm, Kees Cook wrote:
> > On Fri, Feb 14, 2025 at 07:39:20PM +0000, Andrew Cooper wrote:
> >> Architecturally, FineIBT without FRED seems to be no improvement over
> >> simple IBT.  (I'd love to find some way of hardening the entrypoints,
> >> but I can't see a robust way of doing so.)
> > If you're just looking at IBT, yes. But kCFI (with or without IBT,
> > but without FineIBT) will do hash checking at the call site, which
> > should make it impossible to reach the entrypoints from an indirect call
> > in the first place, as they have no hash preceding them.
> >
> >> However, micro-architecturally, FineIBT is still far better than simple
> >> IBT for speculation issue, seeing as Intel keep on staunchly refusing to
> >> turn off the indirect predictors by default like AMD do.
> >>
> >> A security conscious user ought to be using FineIBT for this, given a
> >> choice, even if it's not perfect in other regards.
> > A security conscious user should use kCFI without FineIBT. :) But I
> > think we might be thinking about different elements of security. I am
> > focusing on control flow, and I think you're considering speculation?
> 
> True.  The security realist knows they're dammed either way, and gets a
> stiff drink instead.

I don't know how any of our livers survive. :)

-- 
Kees Cook