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

[Sean Christopherson]

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