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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Dave Martin <Dave.Martin@arm.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, X86 ML <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>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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 <ab1a3344-60f4-9b9d-81d4-e6538fdcafcf@intel.com>
 <08c91835-8486-9da5-a7d1-75e716fc5d36@intel.com>
 <a881837d-c844-30e8-a614-8b92be814ef6@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <cbec8861-8722-ec31-2c02-1cfed20255eb@intel.com>
Date:   Wed, 9 Sep 2020 16:25:16 -0700
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On 9/9/2020 4:11 PM, Dave Hansen wrote:
> On 9/9/20 4:07 PM, Yu, Yu-cheng wrote:
>> What if a writable mapping is passed to madvise(MADV_SHSTK)?  Should
>> that be rejected?
> 
> It doesn't matter to me.  Even if it's readable, it _stops_ being even
> directly readable after it's a shadow stack, right?  I don't think
> writes are special in any way.  If anything, we *want* it to be writable
> because that indicates that it can be written to, and we will want to
> write to it soon.
> 
But in a PROT_WRITE mapping, all the pte's have _PAGE_BIT_RW set.  To 
change them to shadow stack, we need to clear that bit from the pte's.
That will be like mprotect_fixup()/change_protection_range().