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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>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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 <CAMe9rOqt9kbqERC8U1+K-LiDyNYuuuz3TX++DChrRJwr5ajt6Q@mail.gmail.com>
 <20200901102758.GY6642@arm.com>
 <c91bbad8-9e45-724b-4526-fe3674310c57@intel.com>
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 <08c91835-8486-9da5-a7d1-75e716fc5d36@intel.com>
 <a881837d-c844-30e8-a614-8b92be814ef6@intel.com>
 <cbec8861-8722-ec31-2c02-1cfed20255eb@intel.com>
 <b3379d26-d8a7-deb7-59f1-c994bb297dcb@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <fe6774f4-2554-9c0c-f1b8-110bb9398324@intel.com>
Date:   Wed, 9 Sep 2020 16:45:59 -0700
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On 9/9/2020 4:29 PM, Dave Hansen wrote:
> On 9/9/20 4:25 PM, Yu, Yu-cheng wrote:
>> 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().
> 
> The page table hardware bits don't matter.  The user-visible protection
> effects matter.
> 
> For instance, we have PROT_EXEC, which *CLEARS* a hardware NX PTE bit.
> The PROT_ permissions are independent of the hardware.

Same for shadow stack here.  We consider shadow stack "writable", but we 
want to clear _PAGE_BIT_RW, which is set for the PTEs of the other type 
of "writable" mapping.  To change a writable data mapping to a writable 
shadow stack mapping, we need to call change_protection_range().

> 
> I don't think the interface should be influenced at *all* by what whacko
> PTE bit combinations we have to set to get the behavior.
>