linux-kernel - Re: [PATCH 34/35] x86/cet/shstk: Support wrss for userspace

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Message-ID: <87a6fb7nih.fsf@oldenburg.str.redhat.com>
Date:   Mon, 31 Jan 2022 19:45:10 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 34/35] x86/cet/shstk: Support wrss for userspace

* H. J. Lu:

> On Sun, Jan 30, 2022 at 11:57 PM Florian Weimer <fweimer@...hat.com> wrote:
>>
>> * Rick Edgecombe:
>>
>> > For the current shadow stack implementation, shadow stacks contents cannot
>> > be arbitrarily provisioned with data. This property helps apps protect
>> > themselves better, but also restricts any potential apps that may want to
>> > do exotic things at the expense of a little security.
>> >
>> > The x86 shadow stack feature introduces a new instruction, wrss, which
>> > can be enabled to write directly to shadow stack permissioned memory from
>> > userspace. Allow it to get enabled via the prctl interface.
>>
>> Why can't this be turned on unconditionally?
>
> WRSS can be a security risk since it defeats the whole purpose of
> Shadow Stack.  If an application needs to write to shadow stack,
> it can make a syscall to enable it.  After the CET patches are checked
> in Linux kernel, I will make a proposal to allow applications or shared
> libraries to opt-in WRSS through a linker option, a compiler option or
> a function attribute.

Ahh, that makes sense.  I assumed that without WRSS, the default was to
allow plain writes. 8-)

Thanks,
Florian