linux-kernel - Re: [RFC PATCH v2 22/27] x86/cet/ibt: User-mode indirect branch tracking support

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Message-ID: <aa89221e-8900-12f5-d40e-d1db78b11845@linux.intel.com>
Date:   Fri, 13 Jul 2018 11:05:23 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omiun.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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 22/27] x86/cet/ibt: User-mode indirect branch
 tracking support

On 07/13/2018 10:56 AM, Yu-cheng Yu wrote:
>>> GLIBC does the bitmap setup.  It sets bits in there.
>>> I thought you wanted a smaller bitmap?  One way is forcing legacy libs
>>> to low address, or not having the bitmap at all, i.e. turn IBT off.
>> I'm concerned with two things:
>> 1. the virtual address space consumption, especially the *default* case
>>    which will be apps using 4-level address space amounts, but having
>>    5-level-sized tables.
>> 2. the driving a truck-sized hole in the address space limits
>>
>> You can force legacy libs to low addresses, but you can't stop anyone
>> from putting code into a high address *later*, at least with the code we
>> have today.
> So we will always reserve a big space for all CET tasks?

Yes.  You either hard-restrict the address space (which we can't do
currently) or you reserve a big space.

> Currently if an application does dlopen() a legacy lib, it will have only
> partial IBT protection and no SHSTK.  Do we want to consider simply turning
> off IBT in that case?

I don't know.  I honestly don't understand the threat model enough to
give you a good answer.  Is there background on this in the docs?