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

[Yu, Yu-cheng]

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 <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. */

Thanks, I will fix that.

Yu-cheng