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

[Nathaniel McCallum <npmccallum@redhat.com>]

On Wed, Mar 18, 2020 at 7:41 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Sat, Mar 14, 2020 at 10:10:26AM -0400, Nathaniel McCallum wrote:
> > On Fri, Mar 13, 2020 at 6:08 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > > > > > 4. sub/add to %rsp rather than save/restore
> > > > >
> > > > > Can you elaborate on why you want to sub/add to %rsp instead of having the
> > > > > enclave unwind the stack?  Preserving %rsp across EEXIT/ERESUME seems more
> > > > > in line with function call semantics, which I assume is desirable?  E.g.
> > > > >
> > > > >   push param3
> > > > >   push param2
> > > > >   push param1
> > > > >
> > > > >   enclu[EEXIT]
> > > > >
> > > > >   add $0x18, %rsp
> > > >
> > > > Before enclave EEXIT, the enclave restores %rsp to the value it had
> > > > before EENTER was called. Then it pushes additional output arguments
> > > > onto the stack. The enclave calls EENCLU[EEXIT].
> > > >
> > > > We are now in __vdso...() on the way back to the caller. However, %rsp
> > > > has a different value than we entered the function with. This breaks
> > > > x86_64 ABI, obviously. The handler needs to fix this up: how does it
> > > > do so?
>
> Circling back to this request, because I just realized that the above is
> handled by saving %rsp into %rbp and requiring the enclave and handler
> to preserve %rbp at all times.
>
> So the below discussion on making the %rsp adjustment relative is moot,
> at least with respect to getting out of __vdso() if the enclave has mucked
> with the untrusted stack.

You're right. __vdso() will always restore the caller's stack via the
leave instruction at .Lout. So no change is necessary.

> > > > In the current code, __vdso..() saves the value of %rsp, calls the
> > > > handler and then restores %rsp. The handler can fix up the stack by
> > > > setting the correct value to %rbx and returning without restoring it.
> > >
> > > Ah, you're referring to the patch where the handler decides to return all
> > > the way back to the caller of __vdso...().
> > >
> > > > But this requires internal knowledge of the __vdso...() function,
> > > > which could theoretically change in the future.
> > > >
> > > > If instead the __vdso...() only did add/sub, then the handler could do:
> > > > 1. pop return address
> > > > 2. pop handler stack params
> > > > 3. pop enclave additional output stack params
> > > > 4. push handler stack params
> > > > 5. push return address
>
> Per above, this is unnecessary when returning to the caller of __vdso().
> It would be necessary if the enclave wasn't smart enough to do it's own
> stack cleanup, but that seems like a very bizarre contract between the
> enclave and its runtime.
>
> The caveat is if %rbx is saved/restored by __vdso().  If we want a
> traditional frame pointer, then %rbx would be restored from the stack
> before %rsp itself is restored (from %rbp), in which case the exit handler
> would need to adjust %rsp using a sequence similar to what you listed
> above.
>
> If __vdso() uses a non-standard frame pointer, e.g.
>
>   push %rbp
>   push %rbx
>   mov  %rsp, %rbp
>
> then %rbx would come off the stack after %rsp is restored from %rbp, i.e.
> would be guaranteed to be restored to the pre-EENTER value (unless the
> enclave or handler mucks with %rbp).
>
> Anyways, we can discuss how to implement the frame pointer in the context
> of the patches, just wanted to point this out here for completeness.
>
> > > > 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
> > >
> > >
> > > That's reasonable, let's the handler play more games with minimal overhead.
> >
> > Yes, exactly!
> >
> > > > > > That would make this a very usable and fast interface without
> > > > > > sacrificing any of its current power.
> > > > >
> > > >
> > >
> >
>