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Subject: Re: [PATCH v26 0/9] Control-flow Enforcement: Indirect Branch
 Tracking
To:     Andy Lutomirski <luto@kernel.org>, "H.J. Lu" <hjl.tools@gmail.com>
Cc:     David Laight <David.Laight@aculab.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>
References: <20210427204720.25007-1-yu-cheng.yu@intel.com>
 <0e03c50ea05440209d620971b9db4f29@AcuMS.aculab.com>
 <CALCETrUpZfznXzN3Ld33DMvQcHD2ACnhYf9KdP+5-xXuX_pVpA@mail.gmail.com>
 <CAMe9rOp7FauoqQ0vx+ZVPGOE9+ABspheuGLc++Chj_goE5HvWA@mail.gmail.com>
 <CALCETrVHUP9=2kX3aJJugcagsf26W0sLEPsDvVCZNnBmbWrOLQ@mail.gmail.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <9f7dbabe-f369-fef9-a303-53e94c5fa4ad@intel.com>
Date:   Wed, 28 Apr 2021 08:42:48 -0700
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On 4/28/2021 8:14 AM, Andy Lutomirski wrote:
> 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?

David, do you mean kernel loadable drivers here?  Do not worry about it 
for now, since shadow stack/ibt is not enabled for kernel-mode yet.

>>>>
>>>> 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?
> 

Let me clarify.  IBT bitmap isn't used at all.  How dlopen() works 
depends entirely on the tunable of glibc.cpu.x86_ibt.  There are three 
values: on, off, permissive.  On is always on, and off is always off, 
regardless of the .so having ibt or not.  The default is "permissive," 
which turns off ibt upon dlopen a legacy .so.  I hope this also answers 
Andy's question above.

Yu-cheng