Re: [PATCH v26 0/9] Control-flow Enforcement: Indirect Branch Tracking

Yu, Yu-cheng
2021-04-28T15:43:08Z

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