Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 2 Dec 2022 18:52:32 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, 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>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        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>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        John Allen <john.allen@amd.com>, kcc@google.com,
        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
        dethoma@microsoft.com, akpm@linux-foundation.org,
        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com
Subject: Re: [PATCH v4 31/39] x86/shstk: Support wrss for userspace
Message-ID: <202212021852.88C57CF544@keescook>
References: <20221203003606.6838-1-rick.p.edgecombe@intel.com>
 <20221203003606.6838-32-rick.p.edgecombe@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221203003606.6838-32-rick.p.edgecombe@intel.com>
Precedence: bulk

On Fri, Dec 02, 2022 at 04:35:58PM -0800, Rick Edgecombe wrote:
> For the current shadow stack implementation, shadow stacks contents can't
> easily be provisioned with arbitrary 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.
> 
> Only enable the userspace wrss instruction, which allows writes to
> userspace shadow stacks from userspace. Do not allow it to be enabled
> independently of shadow stack, as HW does not support using WRSS when
> shadow stack is disabled.
> 
> From a fault handler perspective, WRSS will behave very similar to WRUSS,
> which is treated like a user access from a #PF err code perspective.
> 
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook