Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

[Yu, Yu-cheng]

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!

--
Yu-cheng

> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > ---
> >   arch/x86/entry/vdso/vsgx.S | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/entry/vdso/vsgx.S b/arch/x86/entry/vdso/vsgx.S
> > index 86a0e94f68df..a70d4d09f713 100644
> > --- a/arch/x86/entry/vdso/vsgx.S
> > +++ b/arch/x86/entry/vdso/vsgx.S
> > @@ -27,6 +27,9 @@
> >   SYM_FUNC_START(__vdso_sgx_enter_enclave)
> >   	/* Prolog */
> >   	.cfi_startproc
> > +#ifdef CONFIG_X86_CET
> > +	endbr64
> > +#endif
> >   	push	%rbp
> >   	.cfi_adjust_cfa_offset	8
> >   	.cfi_rel_offset		%rbp, 0
> > --
> > 2.21.0
>
> /Jarkko