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

[Sean Christopherson <seanjc@google.com>]

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
>