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From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Subject: 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>,
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        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
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        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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Message-ID: <35af3052-324f-06e3-5092-ac6d435f1725@intel.com>
Date:   Wed, 16 Sep 2020 12:25:11 -0700
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On 9/16/2020 6:52 AM, Andy Lutomirski wrote:
> On Mon, Sep 14, 2020 at 2:14 PM Dave Hansen <dave.hansen@intel.com> wrote:
>>
>> On 9/14/20 11:31 AM, Andy Lutomirski wrote:
>>> 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.
>>
>> What's odd about it?  If you're a vfork()'d child, you can't touch the
>> stack at all, right?  If you do, you or your parent will probably die a
>> horrible death.
>>
> 
> An evil program could vfork(), have the child do a bunch of returns
> and a bunch of calls, and exit.  The net effect would be to change the
> parent's shadow stack contents.  In a sufficiently strict model, this
> is potentially problematic.

When a vfork child returns, its parent's shadow stack pointer is where 
it was before the child starts.  To move the shadow stack pointer and 
re-use the content left by the child, the parent needs to use CALL, RET, 
INCSSP, or RSTORSSP.  This seems to be difficult.

> 
> The question is: how much do we want to protect userspace from itself?
 >

If any issue comes up, people can always find ways to counter it.

> --Andy
>