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Date:   Wed, 31 Aug 2022 15:04:20 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     linux-kernel@vger.kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, llvm@lists.linux.dev
Subject: Re: [PATCH v4 00/21] KCFI support
Message-ID: <202208311503.62B2C1BC@keescook>
References: <20220830233129.30610-1-samitolvanen@google.com>
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On Tue, Aug 30, 2022 at 04:31:08PM -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 19 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 21 add
> arch-specific error handlers.
> 
> To test this series, you'll a ToT Clang toolchain. The series is
> also available in GitHub:
> 
>   https://github.com/samitolvanen/linux/commits/kcfi-v4

This happily runs through all my initial testing hoops. :)

Tested-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook