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Date:   Mon, 25 Jan 2021 15:19:29 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
        Mark Brown <broonie@kernel.org>,
        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>
Subject: Re: [RFC PATCH 00/17] objtool: add base support for arm64
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On Fri, Jan 22, 2021 at 10:43:09PM +0100, Ard Biesheuvel wrote:
> 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.

It's quite relevant to this thread.  There's no need to consider merging
Julien's patches if you have a better approach.  Why not discuss it
here?  I'm also interested in the answers to Madhavan's questions.

-- 
Josh