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    SubjectRe: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions
    On Tue, Dec 11, 2018 at 12:04:15PM -0800, Andy Lutomirski wrote:
    > On Tue, Dec 11, 2018 at 11:31 AM Sean Christopherson
    > <sean.j.christopherson@intel.com> wrote:
    > >
    > > On Fri, Dec 07, 2018 at 03:33:57PM -0800, Andy Lutomirski wrote:
    > > > On Fri, Dec 7, 2018 at 1:26 PM Sean Christopherson
    > > > <sean.j.christopherson@intel.com> wrote:
    > > > >
    > > > > Running a checksum on the stack for every exit doesn't seem like it'd
    > > > > be worth the effort, especially since this type of bug should be quite
    > > > > rare, at least in production environments.
    > > > >
    > > > > If we want to pursue the checksum idea I think the easiest approach
    > > > > would be to combine it with an exit_handler and do a simple check on
    > > > > the handler. It'd be minimal overhead in the fast path and would flag
    > > > > cases where invoking exit_handle() would explode, while deferring all
    > > > > other checks to the user.
    > > >
    > > > How about this variant?
    > > >
    > > > #define MAGIC 0xaaaabbbbccccddddul
    > > > #define RETADDR_HASH ((unsigned long)__builtin_return_address(0) ^ MAGIC)
    > > >
    > > > void foo(void)
    > > > {
    > > > volatile unsigned long hash = RETADDR_HASH;
    > > >
    > > > /* placeholder for your actual code */
    > > > asm volatile ("nop");
    > > >
    > > > if (hash != RETADDR_HASH)
    > > > asm volatile ("ud2");
    > > > }
    > > >
    > > > But I have a real argument for dropping exit_handler: in this new age
    > > > of Spectre, the indirect call is a retpoline, and it's therefore quite
    > > > slow.
    > >
    > > Technically slower, but would the extra CALL+RET pair even be noticeable
    > > in the grand scheme of SGX?
    >
    > But it's CALL, CALL, MOV to overwrite return address, intentionally
    > midpredicted RET, and RET because Spectre. That whole sequence seems
    > to be several tens of cycles, so it's a lot worse than just CALL+RET.
    > Whether it's noticeable overall is a fair question, though.

    I was thinking of the case where the handler re-entered the enclave vs.
    leaving and re-calling the vDSO, which would be RET+CALL and some other
    stuff.

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