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

Sean Christopherson
2022-10-12T16:35:12Z

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).

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?

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;

>
> 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
>