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Subject: Re: [PATCH v18 21/25] x86/cet/shstk: Handle signals for shadow stack
To:     Dave Hansen <dave.hansen@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
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        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>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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        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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
References: <20210127212524.10188-1-yu-cheng.yu@intel.com>
 <20210127212524.10188-22-yu-cheng.yu@intel.com>
 <09da02c5-c6fb-878e-ad34-222f3a152460@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <21462baa-8924-f005-33e1-f5fa36ed9942@intel.com>
Date:   Mon, 1 Feb 2021 14:58:19 -0800
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On 2/1/2021 2:53 PM, Dave Hansen wrote:
> On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
>> To deliver a signal, create a shadow stack restore token and put a restore
>> token and the signal restorer address on the shadow stack.  For sigreturn,
>> verify the token and restore the shadow stack pointer.
>>
>> Introduce WRUSS, which is a kernel-mode instruction but writes directly to
>> user shadow stack.  It is used to construct the user signal stack as
>> described above.
>>
>> Introduce a signal context extension struct 'sc_ext', which is used to save
>> shadow stack restore token address and WAIT_ENDBR status.  WAIT_ENDBR will
>> be introduced later in the Indirect Branch Tracking (IBT) series, but add
>> that into sc_ext now to keep the struct stable in case the IBT series is
>> applied later.
> 
> This changelog needs some work.  It's got a lot of "what" and not enough
> "why".
> 
> Why do we need a token?
> What function does it serve?
> What does it protect against?
> Why do we need a signal context extension?
> 

I will update it.

--
Yu-cheng