linux-kernel - Re: [PATCH] hardening: Refresh KCFI options, add some more

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Date: Wed, 1 May 2024 12:27:38 -0700
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...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>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-hardening@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hardening: Refresh KCFI options, add some more

On Wed, May 01, 2024 at 01:06:14PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 30, 2024 at 10:48:36AM -0700, Kees Cook wrote:
> > On Tue, Apr 30, 2024 at 11:21:40AM +0200, Peter Zijlstra wrote:
> > > On Fri, Apr 26, 2024 at 03:29:44PM -0700, Kees Cook wrote:
> > > 
> > > > - CONFIG_CFI_CLANG=y for x86 and arm64. (And disable FINEIBT since
> > > >   it isn't as secure as straight KCFI.)
> > > 
> > > Oi ?
> > 
> > Same objection I always had[1]: moving the check into the destination
> > means attacks with control over executable memory contents can just omit
> > the check.
> 
> I thought it was game over if you could write arbitrary test anyway?

Yes, an attack having arbitrary write control over an executable memory
area is the place where CFI does get much weaker. But presently, there's
no reason to make it easier. With the hash randomization we have, an
attack needs to locate and read the kcfi_seed, otherwise everything else
is a fixed value.

In the future, when we have viable X^R kernel text, then the trade-off
becomes much more clear: we want FineIBT very very much. :)

> The whole CFI thing was about clobbering data (function pointers to be
> more specific), and both are robust against that.

Yes, if the threat model is limited to arbitrary writes to function
pointers, we're totally good with FineIBT.

-Kees

-- 
Kees Cook