# Re: \[PATCH v24 9/9\] x86/vdso: Add ENDBR to \_\_vdso\_sgx\_enter\_enclave

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-05-24 19:01:40 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/05/1390431
- Thread: https://ratatoskr.run/linux-doc/2021/04/6324705/t.md

```text
On 5/24/2021 11:51 AM, Sean Christopherson wrote:
> On Thu, Apr 01, 2021, Yu-cheng Yu wrote:
>> ENDBR is a special new instruction for the Indirect Branch Tracking (IBT)
>> 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.
>>
>> Add ENDBR to __vdso_sgx_enter_enclave() branch targets.
>                                            ^
>                                            |- indirect
>
> After reading the changelog, I was expecting ENDBR on every label.
>

Sorry about the confusion.

>> Signed-off-by: Yu-cheng Yu <redacted>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Jarkko Sakkinen <jarkko@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> ---
>>   arch/x86/entry/vdso/vsgx.S | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/entry/vdso/vsgx.S b/arch/x86/entry/vdso/vsgx.S
>> index 86a0e94f68df..c63eafa54abd 100644
>> --- a/arch/x86/entry/vdso/vsgx.S
>> +++ b/arch/x86/entry/vdso/vsgx.S
>> @@ -4,6 +4,7 @@
>>   #include <asm/export.h>
>>   #include <asm/errno.h>
>>   #include <asm/enclu.h>
>> +#include <asm/vdso.h>
>>   
>>   #include "extable.h"
>>   
>> @@ -27,6 +28,7 @@
>>   SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   	/* Prolog */
>>   	.cfi_startproc
>> +	ENDBR
>>   	push	%rbp
>>   	.cfi_adjust_cfa_offset	8
>>   	.cfi_rel_offset		%rbp, 0
>> @@ -62,6 +64,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   .Lasync_exit_pointer:
>>   .Lenclu_eenter_eresume:
>>   	enclu
>> +	ENDBR
>
> It would be better to move this below the comment about EEXIT.  As is, it looks
> like a misplaced annotation on the AEP.  The AEP doesn't need ENDBR, it's the
> EEXIT target that needs ENDBR because EEXIT is treated as an indirect branch.
>
> Might also be helpful for future readers to explicitly state in the changelog
> that EEXIT is considered an indirect branch.
>
> I.e.
>
>>   	/* EEXIT jumps here unless the enclave is doing something fancy. */
> 	ENDBR
>>   	mov	SGX_ENCLAVE_OFFSET_OF_RUN(%rbp), %rbx
>> @@ -91,6 +94,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>>   	jmp	.Lout
>>   
>>   .Lhandle_exception:
>> +	ENDBR
>>   	mov	SGX_ENCLAVE_OFFSET_OF_RUN(%rbp), %rbx
>>   
>>   	/* Set the exception info. */

Thanks, I will fix that.

Yu-cheng
```