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

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Date:   Thu, 5 May 2022 13:36:29 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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 00/21] KCFI support

On Wed, May 04, 2022 at 01:17:25PM -0700, Sami Tolvanen wrote:
> On Wed, May 4, 2022 at 9:41 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > Hi Mark,
> >
> > On Wed, May 4, 2022 at 9:18 AM Mark Rutland <mark.rutland@....com> wrote:
> > > I wanted to give this a spin on arm64, but I'm seeing some very odd toolchain
> > > behaviour. I'm not sure if I've done something wrong, or if I'm just hitting an
> > > edge-case, but it looks like using -fsanitize=kcfi causes the toolchain to hit
> > > out-of-memory errors and other issues which look like they could be memory
> > > corruption.
> >
> > Thanks for the detailed bug report! It definitely looks like something
> > is wrong with the recent switch from std::string to Twine in the Clang
> > code. I didn't see this issue when compiling the arm64 kernel, but
> > I'll take a closer look and see if I can reproduce it.
> 
> I was able to reproduce this by turning off assertions in Clang. It
> seems to work fine with -DLLVM_ENABLE_ASSERTIONS=ON. I'll go fix.

FWIW, a `-DLLVM_ENABLE_ASSERTIONS=ON` build also seems to work for me when
building a kernel with CONFIG_CFI_CLANG=y. It's much slower than a regular
Release build, so I'm still waiting for that to finish building a kernel, but
it has gotten much further through the build without issues.

Thanks,
Mark.