Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Xing, Cedric]

On 3/17/2020 3:09 PM, Sean Christopherson wrote:
> On Tue, Mar 17, 2020 at 02:40:34PM -0700, Xing, Cedric wrote:
> > Hi Nathaniel,
> >
> > I reread your email today and thought I might have misunderstood your email
> > earlier. What changes are you asking for exactly? Is that just passing @leaf
> > in %ecx rather than in %eax? If so, I wouldn't have any problem. I agree
> > with you that the resulted API would then be callable from C, even though it
> > wouldn't be able to return back to C due to tampered %rbx. But I think the
> > vDSO API can preserve %rbx too, given it is used by both EENTER and EEXIT
> > (so is unavailable for parameter passing anyway). Alternatively, the C
> > caller can setjmp() to be longjmp()'d back from within the exit handler.
>
> Yep, exactly.  The other proposed change that is fairly straightforward is
> to make the save/restore of %rsp across the exit handler call relative
> instead of absolute, i.e. allow the exit handler to modify %rsp.  I don't
> think this would conflict with the Intel SDK usage model?
>
> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> index 94a8e5f99961..05d54f79b557 100644
> --- a/arch/x86/entry/vdso/vsgx_enter_enclave.S
> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> @@ -139,8 +139,9 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>          /* Pass the untrusted RSP (at exit) to the callback via %rcx. */
>          mov     %rsp, %rcx
>
> -       /* Save the untrusted RSP in %rbx (non-volatile register). */
> +       /* Save the untrusted RSP offset in %rbx (non-volatile register). */
>          mov     %rsp, %rbx
> +       and     $0xf, %rbx
>
>          /*
>           * Align stack per x86_64 ABI. Note, %rsp needs to be 16-byte aligned
> @@ -161,8 +162,8 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
>          mov     0x20(%rbp), %rax
>          call    .Lretpoline
>
> -       /* Restore %rsp to its post-exit value. */
> -       mov     %rbx, %rsp
> +       /* Undo the post-exit %rsp adjustment. */
> +       lea     0x20(%rsp,%rbx), %rsp
Yep. Though it looks a bit uncommon, I do think it will work.