linux-kernel - Re: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted

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Date:   Thu, 16 Jan 2020 12:36:01 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted

On Thu, Jan 16, 2020 at 9:45 AM Will Deacon <will@...nel.org> wrote:
>
> On Fri, Dec 06, 2019 at 02:13:47PM -0800, Sami Tolvanen wrote:
> > -0:   b       efi_handle_corrupted_x18        // tail call
> > +0:
> > +#ifdef CONFIG_SHADOW_CALL_STACK
> > +     /*
> > +      * Restore x18 before returning to instrumented code. This is
> > +      * safe because the wrapper is called with preemption disabled and
> > +      * a separate shadow stack is used for interrupts.
> > +      */
> > +     mov     x18, x2
> > +#endif
>
> Why not restore it regardless of CONFIG_SHADOW_CALL_STACK?

The ifdefs are here only because restoring the register without SCS
isn't actually necessary, but I'm fine with dropping them (and editing
the comment) in the next version if you prefer.

Sami