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

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Date:   Fri, 13 Jul 2018 10:56:49 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.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 Wed, 2018-07-11 at 16:16 -0700, Dave Hansen wrote:
> On 07/11/2018 04:00 PM, Yu-cheng Yu wrote:
> > 
> > On Wed, 2018-07-11 at 15:40 -0700, Dave Hansen wrote:
> > > 
> > > On 07/11/2018 03:10 PM, Yu-cheng Yu wrote:
> > > > 
> > > > 
> > > > On Tue, 2018-07-10 at 17:11 -0700, Dave Hansen wrote:
> > > > > 
> > > > > 
> > > > > Is this feature *integral* to shadow stacks?  Or, should it just
> > > > > be
> > > > > in a
> > > > > different series?
> > > > The whole CET series is mostly about SHSTK and only a minority for
> > > > IBT.
> > > > IBT changes cannot be applied by itself without first applying
> > > > SHSTK
> > > > changes.  Would the titles help, e.g. x86/cet/ibt, x86/cet/shstk,
> > > > etc.?
> > > That doesn't really answer what I asked, though.
> > > 
> > > Do shadow stacks *require* IBT?  Or, should we concentrate on merging
> > > shadow stacks themselves first and then do IBT at a later time, in a
> > > different patch series?
> > > 
> > > But, yes, better patch titles would help, although I'm not sure
> > > that's
> > > quite the format that Ingo and Thomas prefer.
> > Shadow stack does not require IBT, but they complement each other.  If
> > we can resolve the legacy bitmap, both features can be merged at the
> > same time.
> As large as this patch set is, I'd really prefer to see you get shadow
> stacks merged and then move on to IBT.  I say separate them.

Ok, separate them.

> 
> > 
> > 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?

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?

Yu-cheng