linux-kernel - Re: [PATCH v6 14/15] arm64: implement Shadow Call Stack

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Message-ID: <CABCJKucnitMPUv+NhZu4bscz9qs1qB9TXR1OP-ychFO0LQ4v_g@mail.gmail.com>
Date:   Thu, 16 Jan 2020 13:57:53 -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 14/15] arm64: implement Shadow Call Stack

On Thu, Jan 16, 2020 at 10:24 AM Will Deacon <will@...nel.org> wrote:
> >       .macro  irq_stack_entry
> >       mov     x19, sp                 // preserve the original sp
> > +#ifdef CONFIG_SHADOW_CALL_STACK
> > +     mov     x20, x18                // preserve the original shadow stack
> > +#endif
>
> Hmm, not sure about corrupting x20 here. Doesn't it hold the PMR value from
> kernel_entry?

You're right, and it's used in el1_irq after irq_handler if
CONFIG_ARM64_PSEUDO_NMI is enabled. Thanks for pointing this out.
Looks like one of x24-x29 should be safe here, and the comment needs
to be updated to explain why x20-x23 shouldn't be corrupted.

Sami