On Fri, Dec 14, 2018 at 07:38:30AM -0800, Sean Christopherson wrote:
> On Fri, Dec 14, 2018 at 07:12:04AM -0800, Sean Christopherson wrote:
> > On Fri, Dec 14, 2018 at 09:55:49AM +0000, Jethro Beekman wrote:
> > > On 2018-12-14 03:01, Sean Christopherson wrote:
> > > >+2: pop %rbx
> > > >+ pop %r12
> > > >+ pop %r13
> > > >+ pop %r14
> > > >+ pop %r15
> > > >+ pop %rbp
> > > >+ ret
> > >
> > > x86-64 ABI requires that you call CLD here (enclave may set it).
> >
> > Ugh. Technically MXCSR and the x87 CW also need to be preserved.
> >
> > What if rather than treating the enclave as hostile we require it to be
> > compliant with the x86-64 ABI like any other function? That would solve
> > the EFLAGS.DF, MXCSR and x87 issues without adding unnecessary overhead.
> > And we wouldn't have to save/restore R12-R15. It'd mean we couldn't use
> > the stack's red zone to hold @regs and @e, but that's poor form anyways.
>
> Grr, except the processor crushes R12-R15, FCW and MXCSR on asynchronous
> exits. But not EFLAGS.DF, that's real helpful.
I can think of three options that are at least somewhat reasonable:
1) Save/restore MXCSR and FCW
+ 100% compliant with the x86-64 ABI
+ Callable from any code
+ Minimal documentation required
- Restoring MXCSR/FCW is likely unnecessary 99% of the time
- Slow
2) Clear EFLAGS.DF but not save/restore MXCSR and FCW
+ Mostly compliant with the x86-64 ABI
+ Callable from any code that doesn't use SIMD registers
- Need to document deviations from x86-64 ABI
3) Require the caller to save/restore everything.
+ Fast
+ Userspace can pass all GPRs to the enclave (minus EAX, RBX and RCX)
- Completely custom ABI
- For all intents and purposes must be called from an assembly wrapper
Option (3) actually isn't all that awful. RCX can be used to pass an
optional pointer to a 'struct sgx_enclave_exception' and we can still
return standard error codes, e.g. -EFAULT.
E.g.:
/**
* __vdso_sgx_enter_enclave() - Enter an SGX enclave
*
* %eax: ENCLU leaf, must be EENTER or ERESUME
* %rbx: TCS, must be non-NULL
* %rcx: Optional pointer to 'struct sgx_enclave_exception'
*
* Return:
* 0 on a clean entry/exit to/from the enclave
* -EINVAL if ENCLU leaf is not allowed or if TCS is NULL
* -EFAULT if ENCLU or the enclave faults
*/
ENTRY(__vdso_sgx_enter_enclave)
/* EENTER <= leaf <= ERESUME */
cmp $0x2, %eax
jb bad_input
cmp $0x3, %eax
ja bad_input
/* TCS must be non-NULL */
test %rbx, %rbx
je bad_input
/* save @exception pointer */
push %rcx
/* load leaf, TCS and AEP for ENCLU */
lea 1f(%rip), %rcx
1: enclu
add 0x8, %rsp
xor %eax, %eax
ret
bad_input:
mov $(-EINVAL), %rax
ret
.pushsection .fixup, "ax"
2: pop %rcx
test %rcx, %rcx
je 3f
mov %eax, EX_LEAF(%rcx)
mov %di, EX_TRAPNR(%rcx)
mov %si, EX_ERROR_CODE(%rcx)
mov %rdx, EX_ADDRESS(%rcx)
3: mov $(-EFAULT), %rax
ret
.popsection
_ASM_VDSO_EXTABLE_HANDLE(1b, 3b)
ENDPROC(__vdso_sgx_enter_enclave)
