Messages in this thread | | | Subject | Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call | From | "Xing, Cedric" <> | Date | Tue, 17 Mar 2020 15:36:57 -0700 |
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On 3/17/2020 3:09 PM, Sean Christopherson wrote: > On Tue, Mar 17, 2020 at 02:40:34PM -0700, Xing, Cedric wrote: >> Hi Nathaniel, >> >> I reread your email today and thought I might have misunderstood your email >> earlier. What changes are you asking for exactly? Is that just passing @leaf >> in %ecx rather than in %eax? If so, I wouldn't have any problem. I agree >> with you that the resulted API would then be callable from C, even though it >> wouldn't be able to return back to C due to tampered %rbx. But I think the >> vDSO API can preserve %rbx too, given it is used by both EENTER and EEXIT >> (so is unavailable for parameter passing anyway). Alternatively, the C >> caller can setjmp() to be longjmp()'d back from within the exit handler. > > Yep, exactly. The other proposed change that is fairly straightforward is > to make the save/restore of %rsp across the exit handler call relative > instead of absolute, i.e. allow the exit handler to modify %rsp. I don't > think this would conflict with the Intel SDK usage model? > > 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 > Yep. Though it looks a bit uncommon, I do think it will work.
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