[RFC PATCH 1/5] KVM: nSVM: Do not use L2's RIP for vmcb02's NextRIP after first L2 VMRUN

[Yosry Ahmed <yosry.ahmed@linux.dev>]

For guests with NRIPS disabled, L1 does not provide NextRIP when running
an L2 with an injected soft interrupt, instead it advances L2's RIP
before running it. KVM uses L2's RIP as the NextRIP in vmcb02 to emulate
a CPU without NRIPS.

However, after L2 runs the first time, NextRIP will be updated by the
CPU and/or KVM, and L2's RIP is no longer the correct value to use in
vmcb02. Hence, after save/restore, do not use L2's RIP if a nested run
is not pending (i.e. L2 has run at least once), use the NextRIP value.

Fixes: cc440cdad5b7 ("KVM: nSVM: implement KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE")
CC: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/nested.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index de90b104a0dd..eebbe00714e3 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -844,14 +844,18 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 	vmcb02->control.event_inj_err       = svm->nested.ctl.event_inj_err;
 
 	/*
-	 * next_rip is consumed on VMRUN as the return address pushed on the
+	 * NextRIP is consumed on VMRUN as the return address pushed on the
 	 * stack for injected soft exceptions/interrupts.  If nrips is exposed
-	 * to L1, take it verbatim from vmcb12.  If nrips is supported in
-	 * hardware but not exposed to L1, stuff the actual L2 RIP to emulate
-	 * what a nrips=0 CPU would do (L1 is responsible for advancing RIP
-	 * prior to injecting the event).
+	 * to L1, take it verbatim from vmcb12.
+	 *
+	 * If nrips is supported in hardware but not exposed to L1, stuff the
+	 * actual L2 RIP to emulate what a nrips=0 CPU would do (L1 is
+	 * responsible for advancing RIP prior to injecting the event). This is
+	 * only the case for the first L2 run after VMRUN. After that (e.g.
+	 * during save/restore), NextRIP is updated by the CPU and/or KVM, and
+	 * the value of the L2 RIP from vmcb12 should not be used.
 	 */
-	if (guest_cpu_cap_has(vcpu, X86_FEATURE_NRIPS))
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_NRIPS) || !svm->nested.nested_run_pending)
 		vmcb02->control.next_rip    = svm->nested.ctl.next_rip;
 	else if (boot_cpu_has(X86_FEATURE_NRIPS))
 		vmcb02->control.next_rip    = vmcb12_rip;
-- 
2.53.0.273.g2a3d683680-goog