# Re: \[PATCH v22 8/8\] x86/vdso: Add ENDBR64 to \_\_vdso\_sgx\_enter\_enclave

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-03-10 23:23:25 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/03/6296005
- Thread: https://ratatoskr.run/linux-doc/2021/03/6295910/t.md

```text
On 3/10/2021 3:20 PM, Dave Hansen wrote:
> On 3/10/21 2:55 PM, Yu, Yu-cheng wrote:
>> On 3/10/2021 2:39 PM, Jarkko Sakkinen wrote:
>>> On Wed, Mar 10, 2021 at 02:05:19PM -0800, Yu-cheng Yu wrote:
>>>> When CET is enabled, __vdso_sgx_enter_enclave() needs an endbr64
>>>> in the beginning of the function.
>>>
>>> OK.
>>>
>>> What you should do is to explain what it does and why it's needed.
>>>
>>
>> The endbr marks a branch target.  Without the "no-track" prefix, if an
>> indirect call/jmp reaches a non-endbr opcode, a control-protection fault
>> is raised.  Usually endbr's are inserted by the compiler.  For assembly,
>> these have to be put in manually.  I will add this in the commit log if
>> there is another revision.  Thanks!
>
> This is close, but it's missing a detail or two that I think is
> important for someone like Jarkko trying to figure out what it means for
> his subsystem or driver.
>
> I'd probably say:
>
> ENDBR is a special new instruction for the Indirect Branch Tracking
> (IBR) component of CET.  IBT prevents attacks by ensuring that (most)
> indirect branches and function calls may only land at ENDBR
> instructions.  Branches that don't follow the rules will result in
> control flow (#CF) exceptions.
>
> ENDBR is a noop when IBT is unsupported or disabled.  Most ENDBR
> instructions are inserted automatically by the compiler, but branch
> targets written in assembly must have ENDBR added manually, like this one.
>

Ok, I will update.  Thanks!

--
Yu-cheng
```