Re: [PATCH] x86/sev-es: Fix SEV-ES OUT/IN immediate opcode vc handling

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 28, 2020, Borislav Petkov wrote:
> On Thu, Dec 17, 2020 at 09:19:13AM -0800, Sean Christopherson wrote:
> > On Wed, Dec 16, 2020, Peter Gonda wrote:
> > > 
> > > The IN and OUT immediate instructions only use an 8-bit immediate. The
> > > current VC handler uses the entire 32-bit immediate value. These
> > > instructions only set the first bytes.
> > > 
> > > Tested with a loop back port with "outb %0,$0xe0". Before the port seen
> > > by KVM was 0xffffffffffffffe0 instead of 0xe0. After the correct port
> > > was seen by KVM and the guests loop back OUT then IN were equal.
> > > 
> > > 
> > > Signed-off-by: Peter Gonda <pgonda@google.com>
> > > Acked-by: David Rientjes <rientjes@google.com>
> > > 
> > > 
> > > ---
> > >  arch/x86/kernel/sev-es-shared.c | 8 ++++++--
> > >  drivers/Makefile                | 1 +
> > >  2 files changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
> > > index 7d04b356d44d..6c790377c55c 100644
> > > --- a/arch/x86/kernel/sev-es-shared.c
> > > +++ b/arch/x86/kernel/sev-es-shared.c
> > > @@ -305,14 +305,14 @@ static enum es_result vc_ioio_exitinfo(struct es_em_ctxt *ctxt, u64 *exitinfo)
> > >  	case 0xe4:
> > >  	case 0xe5:
> > >  		*exitinfo |= IOIO_TYPE_IN;
> > > -		*exitinfo |= (u64)insn->immediate.value << 16;
> > > +		*exitinfo |= insn->immediate.bytes[0] << 16;
> > 
> > Can't we just drop the explicit cast to u64?  Or explicitly cast to u8?  Doesn't
> > really matter, but poking into the backing bytes feels a bit backwards.
> 
> GHCB spec says for IOIO_PROT (exit code 0x7b):
> 
> "SW_EXITINFO1 will be set as documented in AMD64 Architecture
> Programmer’s Manual Volume 2: System Programming, Section 15.10.2"
> 
> That section has "Figure 15-2. EXITINFO1 for IOIO Intercept" where
> [31:16] is the intercepted I/O port.
> 
> Now, insn->immediate.value is a union between a signed int and a
> unsigned char bytes[4] array so in order to be absolutely correct, that
> should use:
> 
> 	insn->immediate.bytes[0]

Eh, casting to u8 is "absolutely correct" as well.  I don't like using bytes[]
because it feels like accessing the raw data as opposed to the end result of
decoding, but it's not a sticking point.

> (u64) cast doesn't matter because *exitinfo = 0 at function entry and
> that is part of the first assignment afterwards.
> 
> I still don't see how that sign extension 0xffffffffffffffe0 can happen,
> though. Maybe something gets added during instruction decoding...

Yes, insn_get_immediate() sets insn->immediate.value directly and sign extends
the imm8 (which is correct, e.g. for "AND r/m64, imm8"). 

	switch (inat_immediate_size(insn->attr)) {
	case INAT_IMM_BYTE:
		insn->immediate.value = get_next(signed char, insn);
		insn->immediate.nbytes = 1;
		break;
	...
	}
> 
> Hmmm.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette