linux-kernel - Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

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Message-ID: <0e505563f7dae3849b57fb327f578f41b760b6f7.camel@intel.com>
Date:   Fri, 07 Jun 2019 13:06:10 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.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@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function

On Fri, 2019-06-07 at 13:00 -0700, Dave Hansen wrote:
> On 6/7/19 12:49 PM, Yu-cheng Yu wrote:
> > > 
> > > This also gives us an excellent opportunity to make it read-only as seen
> > > from
> > > userspace to prevent exploits from just poking it full of ones before
> > > redirecting execution.
> > 
> > GLIBC sets bits only for legacy code, and then makes the bitmap read-
> > only.  That
> > avoids most issues:
> > 
> >   To populate bitmap pages, mprotect() is required.
> >   Reading zero bitmap pages would not waste more physical memory, right?
> 
> Huh, how does glibc know about all possible past and future legacy code
> in the application?

When dlopen() gets a legacy binary and the policy allows that, it will manage
the bitmap:

  If a bitmap has not been created, create one.
  Set bits for the legacy code being loaded.

Yu-cheng