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

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Date:   Tue, 17 May 2022 13:25:58 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <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 <linux-arm-kernel@...ts.infradead.org>,
        llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 00/21] KCFI support

On Tue, May 17, 2022 at 1:58 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

Good catch, I'll fix this.

Sami