Re: RFC: userspace exception fixups

[Sean Christopherson]

On Thu, Nov 01, 2018 at 04:22:55PM -0700, Andy Lutomirski wrote:
> On Thu, Nov 1, 2018 at 2:24 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 1, 2018 at 12:31 PM Rich Felker <dalias@xxxxxxxx> wrote:
> > >
> > > See my other emails in this thread. You would register the *address*
> > > (in TLS) of a function pointer object pointing to the handler, rather
> > > than the function address of the handler. Then switching handler is
> > > just a single store in userspace, no syscalls involved.
> >
> > Yes.
> >
> > And for just EENTER, maybe that's the right model.
> >
> > If we want to generalize it to other thread-synchronous faults, it
> > needs way more information and a list of handlers, but if we limit the
> > thing to _only_ EENTER getting an SGX fault, then a single "this is
> > the fault handler" address is probably the right thing to do.
> 
> It sounds like you're saying that the kernel should know, *before*
> running any user fixup code, whether the fault in question is one that
> wants a fixup.  Sounds reasonable.
> 
> I think it would be nice, but not absolutely necessary, if user code
> didn't need to poke some value into TLS each time it ran a function
> that had a fixup.  With the poke-into-TLS approach, it looks a lot
> like rseq, and rseq doesn't nest very nicely.  I think we really want
> this mechanism to Just Work.  So we could maybe have a syscall that
> associates a list of fixups with a given range of text addresses.  We
> might want the kernel to automatically zap the fixups when the text in
> question is unmapped.

If this is EENTER specific then nesting isn't an issue.  But I don't
see a simple way to restrict the mechanism to EENTER.

What if rather than having userspace register an address for fixup the
kernel instead unconditionally does fixup on the ENCLU opcode?  For
example, skip the instruction and put fault info into some combination
of RDX/RSI/RDI (they're cleared on asynchronous enclave exits).

The decode logic is straightforward since ENCLU doesn't have operands,
we'd just have to eat any ignored prefixes.  The intended convention
for EENTER is to have an ENCLU at the AEX target (to automatically do
ERESUME after INTR, etc...), so this would work regardless of whether
the fault happened on EENTER or in the enclave.  EENTER/ERESUME are
the only ENCLU functions that are allowed outside of an enclave so
there's no danger of accidentally crushing something else.

This way we wouldn't need a VDSO blob and we'd enforce the kernel's
ABI, e.g. a library that tried to use signal handling would go off the
rails when the kernel mucked with the registers.  We could even have
the SGX EPC fault handler return VM_FAULT_SIGBUS if the faulting
instruction isn't ENCLU, e.g. to further enforce that the AEX target
needs to be ENCLU.


Userspace would look something like this:

    mov tcs, %xbx               /* Thread Control Structure address */
    leaq async_exit(%rip), %rcx /* AEX target for EENTER/RESUME */
    mov $SGX_EENTER, %rax       /* EENTER leaf */

async_exit:
    ENCLU

fault_handler:
    <handle fault>

enclave_exit:                   /* EEXIT target */
    <handle enclave request>