linux-kernel - Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

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Message-ID: <c876d0aa-865d-e48d-0b56-f9d66762f869@intel.com>
Date:   Fri, 27 Aug 2021 11:37:34 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-arch@...r.kernel.org>, <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Eugene Syromiatnikov" <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.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>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack
 support

On 8/27/2021 11:21 AM, Borislav Petkov wrote:
> On Fri, Aug 27, 2021 at 11:10:31AM -0700, Yu, Yu-cheng wrote:
>> Because on context switches the whole xstates are switched together,
>> we need to make sure all are in registers.
> 
> There's context switch code which does that already.
> 
> Why would shstk_setup() be responsible for switching the whole extended
> states buffer instead of only the shadow stack stuff only?
> 

Right now, the kernel does lazy restore, and it waits until right before 
a task goes back to ring-3 to restore xstates.  If a task needs to write 
to any xstate registers before that (e.g. for signals), it restores the 
whole xstates first and clears TIF_NEED_FPU_LOAD, which will prevent 
xstates being restored again later.