[PATCH 4.9 61/83] KVM: x86: Dont update RIP or do single-step on faulting emulation

[Greg Kroah-Hartman]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

commit 75ee23b30dc712d80d2421a9a547e7ab6e379b44 upstream.

Don't advance RIP or inject a single-step #DB if emulation signals a
fault.  This logic applies to all state updates that are conditional on
clean retirement of the emulation instruction, e.g. updating RFLAGS was
previously handled by commit 38827dbd3fb85 ("KVM: x86: Do not update
EFLAGS on faulting emulation").

Not advancing RIP is likely a nop, i.e. ctxt->eip isn't updated with
ctxt->_eip until emulation "retires" anyways.  Skipping #DB injection
fixes a bug reported by Andy Lutomirski where a #UD on SYSCALL due to
invalid state with EFLAGS.TF=1 would loop indefinitely due to emulation
overwriting the #UD with #DB and thus restarting the bad SYSCALL over
and over.

Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
Fixes: 663f4c61b803 ("KVM: x86: handle singlestep during emulation")
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kvm/x86.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5823,12 +5823,13 @@ restart:
 		unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
 		toggle_interruptibility(vcpu, ctxt->interruptibility);
 		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
-		kvm_rip_write(vcpu, ctxt->eip);
-		if (r == EMULATE_DONE && ctxt->tf)
-			kvm_vcpu_do_singlestep(vcpu, &r);
 		if (!ctxt->have_exception ||
-		    exception_type(ctxt->exception.vector) == EXCPT_TRAP)
+		    exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
+			kvm_rip_write(vcpu, ctxt->eip);
+			if (r == EMULATE_DONE && ctxt->tf)
+				kvm_vcpu_do_singlestep(vcpu, &r);
 			__kvm_set_rflags(vcpu, ctxt->eflags);
+		}
 
 		/*
 		 * For STI, interrupts are shadowed; so KVM_REQ_EVENT will