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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>
References: <086c73d8-9b06-f074-e315-9964eb666db9@intel.com>
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 <20200901102758.GY6642@arm.com>
 <c91bbad8-9e45-724b-4526-fe3674310c57@intel.com>
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 <46dffdfd-92f8-0f05-6164-945f217b0958@intel.com>
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 <6e1e22a5-1b7f-2783-351e-c8ed2d4893b8@intel.com>
 <5979c58d-a6e3-d14d-df92-72cdeb97298d@intel.com>
 <ab1a3344-60f4-9b9d-81d4-e6538fdcafcf@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <08c91835-8486-9da5-a7d1-75e716fc5d36@intel.com>
Date:   Wed, 9 Sep 2020 16:07:07 -0700
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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?