Re: [PATCH] KVM: x86: Mark transfer type as X86_TRANSFER_RET when loading CS in iret emulation

Hou Wenlong
2022-10-13T06:52:34Z

On Thu, Oct 13, 2022 at 12:34:57AM +0800, Sean Christopherson wrote:
> On Wed, Oct 12, 2022, Hou Wenlong wrote:
> > When loading code segment descriptor in iret instruction emulation, the
> > checks are same as far return instruction emulation, so transfer type
> > should be X86_TRANSFER_RET in __load_segment_descriptor(). Although,
> > only iret in real mode is implemented now, and no checks are actually
> > needed for real mode, it would still be better to mark transfer type as
> > X86_TRANSFER_RET.
>
> It's not strictly a RET though. The RPL vs. DPL checks in __load_segment_descriptor()
> might do the right thing, but there's a rather large pile of stuff IRET can do that
> RET can't (ignoring the fact that KVM doesn't even emulate FAR RET to outer privilege
> levels).
>
Yes, if EFLAGS.NT is set, IRET performs a task switch instead of far return in Protected
Mode.

> And __emulate_int_real() also loads CS with X86_TRANSFER_NONE, i.e. KVM still has
> a weird path to worry about.
>
> Rather than make the IRET case slightly less wrong, what about adding a sanity
> check in __load_segment_descriptor() that KVM doesn't attempt to load CS in Protected
> Mode with X86_TRANSFER_NONE?
>
OK, sanity check is needed if someone wants to implement IRET in Protected Mode but
use load_segment_descriptor().

> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 3b27622d4642..fe735e18c419 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1641,6 +1641,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
> goto exception;
> break;
> case VCPU_SREG_CS:
> + /*
> + * KVM uses "none" when loading CS as part of emulating Real
> + * Mode exceptions and IRET (handled above). In all other
> + * cases, loading CS without a control transfer is a KVM bug.
> + */
> + if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE))
> + goto exception;
> +
> if (!(seg_desc.type & 8))
> goto exception;
>
Do I need to prepare this patch or you will add this directly?

> >
> > No functional change intended.
> >
> > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> > ---
> > arch/x86/kvm/emulate.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 3b27622d4642..5052eb480068 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -2100,6 +2100,7 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
> > X86_EFLAGS_FIXED;
> > unsigned long vm86_mask = X86_EFLAGS_VM | X86_EFLAGS_VIF |
> > X86_EFLAGS_VIP;
> > + u8 cpl = ctxt->ops->cpl(ctxt);
> >
> > /* TODO: Add stack limit check */
> >
> > @@ -2121,7 +2122,8 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
> > if (rc != X86EMUL_CONTINUE)
> > return rc;
> >
> > - rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS);
> > + rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl,
> > + X86_TRANSFER_RET, NULL);
> >
> > if (rc != X86EMUL_CONTINUE)
> > return rc;
> > --
> > 2.31.1
> >