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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Mar 18, 2020 at 05:38:29PM -0700, Xing, Cedric wrote:
> On 3/18/2020 4:40 PM, Sean Christopherson wrote:
> %rbx can always be restored as long as it is saved at a fixed offset from
> %rbp. For example, given the standard prolog below:
> 
> 	push	%rbp
> 	mov	%rsp, %rbp
> 	push	%rbx
> 
> It can be paired with the following standard epilog:
> 
> 	mov	-8(%rbp), %rbx
> 	leave
> 	ret
> 
> Alternatively, given "red zone" of 128 bytes, the following epilog will also
> work:
> 
> 	leave
> 	mov	-0x10(%rsp), %rbx
> 	ret
> 
> In no cases do we have to worry about enclave mucking the stack as long as
> %rbp is preserved.
> 
> >>>>While this is more work, it is standard calling convention work that
> >>>>doesn't require internal knowledge of __vdso..(). Alternatively, if we
> >>>>don't like the extra work, we can document the %rbx hack explicitly
> >>>>into the handler documentation and make it part of the interface. But
> >>>>we need some explicit way for the handler to pop enclave output stack
> >>>>params that doesn't depend on internal knowledge of the __vdso...()
> >>>>invariants.
> >>>
> >>>IIUC, this is what you're suggesting?  Having to align the stack makes this
> >>>a bit annoying, but it's not bad by any means.
> >>>
> >>>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
> >>>
> 
> Per discussion above, this is useful only if the enclave has problem
> cleaning up its own mess left on the untrusted stack, and the exit handler
> wants to EENTER the enclave again by returning to __vdso...(). It sounds
> very uncommon to me, and more like a bug than an expected behavior. Are
> there any existing code doing this or any particular application that needs
> this. If no, I'd say not to do it.

Ya, I'm on the fence as well.  The only counter-argument is that doing:

	push	%rbp
	mov	%rsp, %rbp
	push	%rbx

	...

	pop	%rbx
	leave
	ret

with the relative adjustment would allow the exit handler (or enclave) to
change %rbx.  I'm not saying that is remote sane, but if we're going for
maximum flexibility...

Anyways, patches incoming, let's discuss there.