Re: [PATCH v24 9/9] x86/vdso: Add ENDBR to __vdso_sgx_enter_enclave

[Sean Christopherson]

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.

> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>  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. */
> -- 
> 2.21.0
>