linux-hardening - Re: [PATCH] x86/alternatives: Make FineIBT mode Kconfig selectable

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Date: Wed, 1 May 2024 13:13:22 -0700
From: Kees Cook <keescook@...omium.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.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>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Juergen Gross <jgross@...e.com>, Breno Leitao <leitao@...ian.org>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/alternatives: Make FineIBT mode Kconfig selectable

On Wed, May 01, 2024 at 09:33:14AM -0700, Nathan Chancellor wrote:
> On Tue, Apr 30, 2024 at 05:02:22PM -0700, Kees Cook wrote:
> > Since FineIBT performs checking at the destination, it is weaker against
> > attacks that can construct arbitrary executable memory contents. As such,
> > some system builders want to run with FineIBT disabled by default. Allow
> > the "cfi=kcfi" boot param mode to be selectable through Kconfig via the
> > newly introduced CONFIG_CFI_AUTO_DEFAULT.
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> 
> I verified that flipping the configuration does indeed change the
> default and that 'cfi=' could still be used to override whatever choice
> was made at compile time. This patch was a perfect excuse to put my new
> CET enabled test machine to work.

Heh, yeah. I have my one lonely CET system that is only powered on for
this sort of testing too. ;)

> 
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Tested-by: Nathan Chancellor <nathan@...nel.org>

Thanks!

> CFI_DEFAULT_AUTO reads a little bit better to me personally but I am not
> looking to get into painting today :)

I went with CFI_AUTO_DEFAULT since this seems to be our current pattern
for setting these kinds of boot param defaults. A selection of examples:

arch/Kconfig:config RANDOMIZE_KSTACK_OFFSET_DEFAULT
lib/Kconfig.debug:config CONSOLE_LOGLEVEL_DEFAULT
mm/Kconfig.debug:config DEBUG_PAGEALLOC_ENABLE_DEFAULT

-- 
Kees Cook