linux-kernel - Re: [RFC PATCH 00/17] objtool: add base support for arm64

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Date:   Fri, 22 Jan 2021 18:54:44 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Julien Thierry <jthierry@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Mark Rutland <mark.rutland@....com>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-hardening@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] objtool: add base support for arm64

On Fri, 22 Jan 2021 at 18:44, Mark Brown <broonie@...nel.org> 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.

This reminds me - a while ago, I had a stab at writing a rudimentary
GCC plugin that pushes/pops return addresses to a shadow call stack
pointed to by x18 [0]
I am by no means suggesting that we should rely on a GCC plugin for
this, only that it does seem rather straight-forward for the compiler
to manage a stack with return addresses like that (although the devil
is probably in the details, as usual)

[0] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=arm64-scs-gcc