Messages in this thread | | | Date | Fri, 14 Dec 2018 09:03:11 -0800 | From | Sean Christopherson <> | Subject | Re: [RFC PATCH v4 5/5] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions |
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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)
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