linux-hardening - Re: [PATCH v4 00/21] KCFI support

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Date:   Fri, 2 Sep 2022 09:51:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        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>,
        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: [PATCH v4 00/21] KCFI support

On Thu, Sep 01, 2022 at 05:33:29PM -0700, Sami Tolvanen wrote:

> > I do see a few new objtool warnings as well:
> >
> > vmlinux.o: warning: objtool: apply_relocate_add+0x34: relocation to !ENDBR: memcpy+0x0
> > vmlinux.o: warning: objtool: ___ksymtab+__memcpy+0x0: data relocation to !ENDBR: memcpy+0x0
> > vmlinux.o: warning: objtool: ___ksymtab+memcpy+0x0: data relocation to !ENDBR: memcpy+0x0
> 
> That's interesting. I can only reproduce this warning with
> allmodconfig+LTO, even though the relocation exists in all builds (the
> code makes an indirect call to memcpy) and memcpy (aliased to
> __memcpy) doesn't start with endbr. I'll have to take a closer look at
> why this warning only appears with LTO.

>From just looking at the patches I'd say patch #19 breaks it. IIRC you
forgot to make the SYM_TYPED_FUNC things emit ENDBR.

Look at how x86/asm/linkage.h is overriding SYM_FUNC_START*().

You might have the same bug vs ARM64 BTI, they do the same thing.