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

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Date:   Wed, 9 Sep 2020 16:07:07 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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 9/9/2020 3:59 PM, Dave Hansen wrote:
> On 9/9/20 3:08 PM, Yu, Yu-cheng wrote:
>> After looking at this more, I found the changes are more similar to
>> mprotect() than madvise().  We are going to change an anonymous mapping
>> to a read-only mapping, and add the VM_SHSTK flag to it.  Would an
>> x86-specific mprotect(PROT_SHSTK) make more sense?
>>
>> One alternative would be requiring a read-only mapping for
>> madvise(MADV_SHSTK).  But that is inconvenient for the application.
> 
> Why?  It's just:
> 
> 	mmap()/malloc();
> 	mprotect(PROT_READ);
> 	madvise(MADV_SHSTK);
> 
> vs.
> 
> 	mmap()/malloc();
> 	mprotect(PROT_SHSTK);
> 
> I'm not sure a single syscall counts as inconvenient.
> 
> I don't quite think we should use a PROT_ bit for this.  It seems like
> the kind of thing that could be fragile and break existing expectations.
>   I don't care _that_ strongly though.
> 
What if a writable mapping is passed to madvise(MADV_SHSTK)?  Should 
that be rejected?