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Date:   Mon, 26 Sep 2022 14:39:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev,
        hjl.tools@...il.com
Subject: Re: [PATCH v5 00/22] KCFI support

On Thu, Sep 08, 2022 at 02:54:42PM -0700, Sami Tolvanen wrote:
> KCFI is a forward-edge control-flow integrity scheme in the upcoming
> Clang 16 release, which is more suitable for kernel use than the
> existing CFI scheme used by CONFIG_CFI_CLANG. KCFI doesn't require
> LTO, doesn't alter function references to point to a jump table, and
> won't break function address equality.
> 
> This series replaces the current arm64 CFI implementation with KCFI
> and adds support for x86_64.
> 
> KCFI requires assembly functions that are indirectly called from C
> code to be annotated with type identifiers. As type information is
> only available in C, the compiler emits expected type identifiers
> into the symbol table, so they can be referenced from assembly
> without having to hardcode type hashes. Patch 6 adds helper macros
> for annotating functions, and patches 9 and 20 add annotations.
> 
> In case of a type mismatch, KCFI always traps. To support error
> handling, the compiler generates a .kcfi_traps section for x86_64,
> which contains the locations of each trap, and for arm64, encodes
> the necessary register information to the ESR. Patches 10 and 22 add
> arch-specific error handlers.
> 
> To test this series, you'll need a ToT Clang toolchain. The series
> is also available pn GitHub:
> 
>   https://github.com/samitolvanen/linux/commits/kcfi-v5

As mentioned at plumbers, my only concern is somewhat excessive use of
CFI_CLANG as oposed to something more compiler neutral. But I suppose
that's something we can cleanup/fix when GCC grows support for this.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@...radead.org>

(as in I ran kCFI + call-depth-tracking + FineIBT on a bunch of
hardware)

HJL, can you look at adding kCFI support to GCC ?

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