linux-hardening - Re: [RFC PATCH v2 00/21] KCFI support

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Date:   Tue, 17 May 2022 10:57:50 +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
Subject: Re: [RFC PATCH v2 00/21] KCFI support

On Fri, May 13, 2022 at 01:21:38PM -0700, Sami Tolvanen wrote:
> KCFI is a proposed forward-edge control-flow integrity scheme for
> Clang, 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. The latest LLVM patch is here:
> 
>   https://reviews.llvm.org/D119296
> 
> This RFC series replaces the current arm64 CFI implementation with
> KCFI and adds support for x86_64.

You have some weird behaviour vs weak functions (I so hate those)...

100: 0000000000000980     9 FUNC    LOCAL  DEFAULT    2 __cfi_free_initmem
233: 0000000000000989    35 FUNC    WEAK   DEFAULT    2 free_initmem

With the result that on the final link:

   179: 00000000000009b0     9 FUNC    LOCAL  DEFAULT    1 __cfi_free_initmem
  8689: 00000000000007f0     9 FUNC    LOCAL  DEFAULT   65 __cfi_free_initmem
173283: 00000000000007f9   198 FUNC    GLOBAL DEFAULT   65 free_initmem

This is getting me objtool issues (I'll fix them) but perhaps it's
something you can do something about as well.