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Date: Tue, 30 Apr 2024 14:25:34 -0700
From: Kees Cook <keescook@chromium.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-hardening@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, llvm@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] hardening: Refresh KCFI options, add some more
Message-ID: <202404301424.FF88864115@keescook>
References: <20240426222940.work.884-kees@kernel.org>
 <20240430092140.GE40213@noisy.programming.kicks-ass.net>
 <202404301037.9E34D4811@keescook>
 <202404301409.D3BC98D5@keescook>
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In-Reply-To: <202404301409.D3BC98D5@keescook>

On Tue, Apr 30, 2024 at 02:15:53PM -0700, Kees Cook 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.
> > 
> > But now that I went to go look I see 0c3e806ec0f9 ("x86/cfi: Add boot
> > time hash randomization") is only enabled under FINEIBT... seems better
> > if that were always enabled...
> 
> And FINEIBT actually can't be disabled... :|
> 
> And as it turns out CFI_CLANG doesn't work at all on v6.9...
> 
> [    0.587220] no CFI hash found at: __call_sites+0x339a8/0x34450 ffffffffac20cef8 00 00 00 00 00
> [    0.588226] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:1182 __apply_fineibt+0x7a9/0x820
> ...
> [    0.619220] SMP alternatives: Something went horribly wrong trying to rewrite the CFI implementation.
> *hang*

Welcome to my TED talk.

I'll keep debugging this: boots fine on v6.8, but booting v6.9 with
defconfig+CFI_CLANG _does_ work, so something in my .config broke it...

-- 
Kees Cook