Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20220130211838.8382-1-rick.p.edgecombe@intel.com>
 <20220130211838.8382-35-rick.p.edgecombe@intel.com> <87wnig8hj6.fsf@oldenburg.str.redhat.com>
In-Reply-To: <87wnig8hj6.fsf@oldenburg.str.redhat.com>
From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Mon, 31 Jan 2022 10:26:49 -0800
Message-ID: <CAMe9rOrVvjL1F3UgOWL-gAGRyyiG6r20TWUusEUFhZMEEAjH7w@mail.gmail.com>
Subject: Re: [PATCH 34/35] x86/cet/shstk: Support wrss for userspace
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
        Kostya Serebryany <kcc@google.com>,
        Stephane Eranian <eranian@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Sun, Jan 30, 2022 at 11:57 PM Florian Weimer <fweimer@redhat.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.

-- 
H.J.