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

[Sean Christopherson]

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?