Re: [PATCH v3 25/75] x86/sev-es: Add support for handling IOIO exceptions

Sean Christopherson
2020-06-03T23:07:16Z

On Wed, Jun 03, 2020 at 04:23:25PM +0200, Joerg Roedel wrote:
> > > + */
> > > + io_bytes = (exit_info_1 >> 4) & 0x7;
> > > + ghcb_count = sizeof(ghcb->shared_buffer) / io_bytes;
> > > +
> > > + op_count = (exit_info_1 & IOIO_REP) ? regs->cx : 1;
> > > + exit_info_2 = min(op_count, ghcb_count);
> > > + exit_bytes = exit_info_2 * io_bytes;
> > > +
> > > + es_base = insn_get_seg_base(ctxt->regs, INAT_SEG_REG_ES);
> > > +
> > > + if (!(exit_info_1 & IOIO_TYPE_IN)) {
> > > + ret = vc_insn_string_read(ctxt,
> > > + (void *)(es_base + regs->si),
> >
> > SEV(-ES) is 64-bit only, why bother with the es_base charade?
>
> User-space can also cause IOIO #VC exceptions, and user-space can be
> 32-bit legacy code with segments, so es_base has to be taken into
> account.

Is there actually a use case for this? Exposing port IO to userspace
doesn't exactly improve security.

Given that i386 ABI requires EFLAGS.DF=0 upon function entry/exit, i.e. is
the de facto default, the DF bug implies this hasn't been tested. And I
don't see how this could possibly have worked for SEV given that the kernel
unrolls string I/O because the VMM can't emulate string I/O. Presumably
someone would have complained if they "needed" to run legacy crud. The
host and guest obviously need major updates, so supporting e.g. DPDK with
legacy virtio seems rather silly.

> > > + ghcb->shared_buffer, io_bytes,
> > > + exit_info_2, df);
> >
> > df handling is busted, it's aways non-zero. Same goes for the SI/DI
> > adjustments below.
>
> Right, this is fixed now.
>
> > Batching the memory accesses and I/O accesses separately is technically
> > wrong, e.g. a #DB on a memory access will result in bogus data being shown
> > in the debugger. In practice it seems unlikely to matter, but I'm curious
> > as to why string I/O is supported in the first place. I didn't think there
> > was that much string I/O in the kernel?
>
> True, #DBs won't be correct anymore. Currently debugging is not
> supported in SEV-ES guests anyway, but if it is supported the #DB
> exception would happen in the #VC handler and not on the original
> instruction.

As in, the guest can't debug itself? Or the host can't debug the guest?