Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Jarkko Sakkinen]

On Fri, Sep 25, 2020 at 07:23:59PM +0100, Andrew Cooper wrote:
> On 15/09/2020 12:28, Jarkko Sakkinen wrote:
> > diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> > new file mode 100644
> > index 000000000000..adbd59d41517
> > --- /dev/null
> > +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> > @@ -0,0 +1,157 @@
> > +SYM_FUNC_START(__vdso_sgx_enter_enclave)
> > <snip>
> > +.Lretpoline:
> > +	call	2f
> > +1:	pause
> > +	lfence
> > +	jmp	1b
> > +2:	mov	%rax, (%rsp)
> > +	ret
> 
> I hate to throw further spanners in the work, but this is not compatible
> with CET, and the user shadow stack work in progress.

CET goes beyond my expertise. Can you describe, at least rudimentary,
how this code is not compatible?

I know CET only conceptual level (separate stack holding return
addresses as an measure against return oriented programming (ROP)).

> Whichever of these two large series lands first is going to inflict
> fixing this problem on the other.
> 
> As the vdso text is global (to a first approximation), it must not be a
> retpoline if any other process is liable to want to use CET-SS.

Why is that?

> If the retpoline really does need to stay, then the vdso probably needs
> to gain suitable __x86_indirect_thunk_%reg thunks which are patched at
> boot based on the system properties.
> 
> ~Andrew

aka without CET it is patched? With CET, not?

/Jarkko