linux-kernel - Re: [PATCH v5 11/14] arm64: efi: restore x18 if it was corrupted

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Date:   Thu, 7 Nov 2019 11:51:38 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 11/14] arm64: efi: restore x18 if it was corrupted

On Wed, 6 Nov 2019 at 05:46, Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Wed, Nov 6, 2019 at 12:56 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > If we detect a corrupted x18 and SCS is enabled, restore the register
> > before jumping back to instrumented code. This is safe, because the
> > wrapper is called with preemption disabled and a separate shadow stack
> > is used for interrupt handling.
>
> In case you do v6: I think putting the explanation about why this is
> safe in the existing comment would be best given it is justifying a
> subtlety of the code rather than the change itself. Ard?
>

Agreed, but only if you have to respin for other reasons.