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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 22 Jan 2021 22:43:09 +0100
Message-ID: <CAMj1kXFr0wvx-hG6nBY4ibju9ww4x0CGhQber3MZQ2ZZn9LHWw@mail.gmail.com>
Subject: Re: [RFC PATCH 00/17] objtool: add base support for arm64
To:     "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc:     Mark Brown <broonie@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Julien Thierry <jthierry@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        linux-hardening@vger.kernel.org, live-patching@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Kees Cook <keescook@chromium.org>
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On Fri, 22 Jan 2021 at 22:15, Madhavan T. Venkataraman
<madvenka@linux.microsoft.com> wrote:
>
>
>
> On 1/22/21 11:43 AM, Mark Brown wrote:
> > On Thu, Jan 21, 2021 at 12:54:52PM -0600, Josh Poimboeuf wrote:
> >
> >> 2) The shadow stack idea sounds promising -- how hard would it be to
> >>    make a prototype reliable unwinder?
> >
> > In theory it doesn't look too hard and I can't see a particular reason
> > not to try doing this - there's going to be edge cases but hopefully for
> > reliable stack trace they're all in areas where we would be happy to
> > just decide the stack isn't reliable anyway, things like nesting which
> > allocates separate shadow stacks for each nested level for example.
> > I'll take a look.
> >
>
> I am a new comer to this discussion and I am learning. Just have some
> questions. Pardon me if they are obvious or if they have already been
> asked and answered.
>
> Doesn't Clang already have support for a shadow stack implementation for ARM64?
> We could take a look at how Clang does it.
>
> Will there not be a significant performance hit? May be, some of it can be
> mitigated by using a parallel shadow stack rather than a compact one.
>
> Are there any longjmp style situations in the kernel where the stack is
> unwound by several frames? In these cases, the shadow stack must be unwound
> accordingly.
>

Hello Madhavan,

Let's discuss the details of shadow call stacks on a separate thread,
instead of further hijacking Julien's series.