On Fri, Mar 13, 2020 at 6:08 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > On Fri, Mar 13, 2020 at 04:14:01PM -0400, Nathaniel McCallum wrote: > > On Fri, Mar 13, 2020 at 2:45 PM Sean Christopherson > > <sean.j.christopherson@xxxxxxxxx> wrote: > > > This doesn't compromise the ability to treat __vsdo...() like ENCLU if > > > > you need the full power. But it does make it significantly easier to > > > > consume when you don't have special needs. So as I see it, __vdso...() > > > > should: > > > > > > > > 1. preserve %rbx > > > > 2. take leaf in %rcx > > > > 3. gain a void* stack param which is passed to the handler > > > > > > Unless I'm misunderstanding the request, this already exists. %rsp at the > > > time of EEXIT is passed to the handler. > > > > Sorry, different stack parameter. I mean this: > > > > typedef int (*sgx_enclave_exit_handler_t)( > > long rdi, > > long rsi, > > long rdx, > > long ursp, > > long r8, > > long r9, > > int ret, > > void *tcs, > > struct sgx_enclave_exception *e, > > void *misc > > ); > > > > int __vdso_sgx_enter_enclave( > > long rdi, > > long rsi, > > long rdx, > > int leaf, > > long r8, > > long r9, > > void *tcs, > > struct sgx_enclave_exception *e, > > void *misc, > > sgx_enclave_exit_handler_t handler > > ); > > > > This is so that the caller of __vdso...() can pass context into the > > handler. > > Hrm, I'm not a fan of adding a param that is only consumed by the handler, > especially when there are multiple alternatives, e.g. fudge the param in > assembly before calling __vdso(), have the enclave supply the context in a > volatile register, etc... Yes, but all of those require assembly. The whole point of this is ergonomics without assembly. Once you can call __vdso() without assembly, having to resort to assembly to make it useful will feel painful. I imagine it would be pretty common to pass something like a jmp_buf reference or a reference to a struct for collecting output stack arguments through misc. > > Note that I've also reordered the stack parameters so that the stack > > order can be reused. > > Ah, ret<->tcs, took me a minute... > > Does preserving tsc->e->misc ordering matter all that much? Not really. I was just trying to aid the reader of the assembly. If there are more important concerns, fine. > __vdso() needs > to manually copy them either way. I ask because putting @misc at the end > would allow implementations that don't use @handler to omit the param (if > I've done my math correctly, which is always a big if). That would make > adding the handler-only param a little more palatable. Fine by me. > > > > 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? > > > > 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 > > > > 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. > > > > > >
