linux-kernel - Re: [PATCH v11 25/25] x86/cet/shstk: Add arch

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Message-ID: <b3379d26-d8a7-deb7-59f1-c994bb297dcb@intel.com>
Date:   Wed, 9 Sep 2020 16:29:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Dave Martin <Dave.Martin@....com>, "H.J. Lu" <hjl.tools@...il.com>,
        Florian Weimer <fweimer@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        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>,
        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>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

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.

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.