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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 28 Apr 2021 08:14:37 -0700
Message-ID: <CALCETrVHUP9=2kX3aJJugcagsf26W0sLEPsDvVCZNnBmbWrOLQ@mail.gmail.com>
Subject: Re: [PATCH v26 0/9] Control-flow Enforcement: Indirect Branch Tracking
To:     "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "linux-api@vger.kernel.org" <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>,
        Florian Weimer <fweimer@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>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>
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On Wed, Apr 28, 2021 at 7:57 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Apr 28, 2021 at 7:52 AM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Wed, Apr 28, 2021 at 7:48 AM David Laight <David.Laight@aculab.com> wrote:
> > >
> > > From: Yu-cheng Yu
> > > > Sent: 27 April 2021 21:47
> > > >
> > > > Control-flow Enforcement (CET) is a new Intel processor feature that blocks
> > > > return/jump-oriented programming attacks.  Details are in "Intel 64 and
> > > > IA-32 Architectures Software Developer's Manual" [1].
> > > ...
> > >
> > > Does this feature require that 'binary blobs' for out of tree drivers
> > > be compiled by a version of gcc that adds the ENDBRA instructions?
> > >
> > > If enabled for userspace, what happens if an old .so is dynamically
> > > loaded?
>
> CET will be disabled by ld.so in this case.

What if a program starts a thread and then dlopens a legacy .so?

>
> > > Or do all userspace programs and libraries have to have been compiled
> > > with the ENDBRA instructions?
>
> Correct.  ld and ld.so check this.
>
> > If you believe that the userspace tooling for the legacy IBT table
> > actually works, then it should just work.  Yu-cheng, etc: how well
> > tested is it?
> >
>
> Legacy IBT bitmap isn't unused since it doesn't cover legacy codes
> generated by legacy JITs.
>

How does ld.so decide whether a legacy JIT is in use?