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References: <20191018161033.261971-1-samitolvanen@google.com>
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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Thu, 7 Nov 2019 11:51:38 +0100
Message-ID: <CAKv+Gu9DD12BPV_jNv9Hjw4oSiZvtdiVVjB-B8WLXCoPL4CA9Q@mail.gmail.com>
Subject: Re: [PATCH v5 11/14] arm64: efi: restore x18 if it was corrupted
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Wed, 6 Nov 2019 at 05:46, Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Wed, Nov 6, 2019 at 12:56 AM Sami Tolvanen <samitolvanen@google.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.