Messages in this thread | | | Date | Mon, 28 Dec 2020 23:36:50 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/sev-es: Fix SEV-ES OUT/IN immediate opcode vc handling |
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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]
instead, to get the port byte only, before shifting it into [31:16]. The (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...
Hmmm.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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