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

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Message-ID: <c875722b-0434-41fe-b375-cc685498c444@dustri.org>
Date: Mon, 29 Jul 2024 14:35:02 +0200
From: jvoisin <julien.voisin@...tri.org>
To: keescook@...omium.org
Cc: ast@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
 jgross@...e.com, jpoimboe@...nel.org, leitao@...ian.org,
 linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
 mingo@...hat.com, nathan@...nel.org, peterz@...radead.org,
 samitolvanen@...gle.com, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH] x86/alternatives: Make FineIBT mode Kconfig selectable

> 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.

I'm confused as why you think that KCFI is stronger/better than FineIBT.
The latter is compatible with execute-only memory, makes use of hardware
support, doesn't need LTO, is faster, … moreover, I don't see why an
attacker able to "construct arbitrary executable memory contents"
wouldn't be able to bypass KCFI as well, since its threat model
(https://github.com/kcfi/docs/blob/master/kCFI_whitepaper.pdf)
explicitly says "We assume an OS that fully implements the W^X policy
[56,58,106] preventing direct code injection in kernel space."