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Subject: Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add
 arch_prctl functions for shadow stack
To:     Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>
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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From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <2d86fa40-3676-62b1-1571-90074ca65971@intel.com>
Date:   Mon, 14 Sep 2020 13:44:50 -0700
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On 9/14/2020 11:31 AM, Andy Lutomirski wrote:
>> On Sep 14, 2020, at 7:50 AM, Dave Hansen <dave.hansen@intel.com> wrote:
>>
>> ﻿On 9/11/20 3:59 PM, Yu-cheng Yu wrote:
>> ...
>>> Here are the changes if we take the mprotect(PROT_SHSTK) approach.
>>> Any comments/suggestions?
>>
>> I still don't like it. :)
>>
>> I'll also be much happier when there's a proper changelog to accompany
>> this which also spells out the alternatives any why they suck so much.
>>
> 
> Let’s take a step back here. Ignoring the precise API, what exactly is
> a shadow stack from the perspective of a Linux user program?
> 
> The simplest answer is that it’s just memory that happens to have
> certain protections.  This enables all kinds of shenanigans.  A
> program could map a memfd twice, once as shadow stack and once as
> non-shadow-stack, and change its control flow.  Similarly, a program
> could mprotect its shadow stack, modify it, and mprotect it back.  In

What if we do the following:

- If the mapping has VM_SHARED, it cannot be turned to shadow stack. 
Shadow stack cannot be shared anyway.

- Only allow an anonymous mapping to be converted to shadow stack, but 
not the other way.

> some threat models, though could be seen as a WRSS bypass.  (Although
> if an attacker can coerce a process to call mprotect(), the game is
> likely mostly over anyway.)
> 
> But we could be more restrictive, or perhaps we could allow user code
> to opt into more restrictions.  For example, we could have shadow
> stacks be special memory that cannot be written from usermode by any
> means other than ptrace() and friends, WRSS, and actual shadow stack
> usage.
> 
> What is the goal?

There primary goal is to allocate/mmap a shadow stack from user space.

> 
> No matter what we do, the effects of calling vfork() are going to be a
> bit odd with SHSTK enabled.  I suppose we could disallow this, but
> that seems likely to cause its own issues.
> 

Do you mean vfork() has issues with call/return?  That is taken care of 
in GLIBC.  Or do you mean it has issues with mprotect(PROT_SHSTK)?