linux-kernel - Re: [PATCH v18 21/25] x86/cet/shstk: Handle signals for shadow stack

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Message-ID: <09da02c5-c6fb-878e-ad34-222f3a152460@intel.com>
Date:   Mon, 1 Feb 2021 14:53:27 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, 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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v18 21/25] x86/cet/shstk: Handle signals for shadow stack

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?