[PATCH v3 1/8] KVM: nSVM: Sync NextRIP to cached vmcb12 after VMRUN of L2

[Yosry Ahmed]

After VMRUN in guest mode, nested_sync_control_from_vmcb02() syncs
fields written by the CPU from vmcb02 to the cached vmcb12. This is
because the cached vmcb12 is used as the authoritative copy of some of
the controls, and is the payload when saving/restoring nested state.

NextRIP is also written by the CPU (in some cases) after VMRUN, but is
not sync'd to the cached vmcb12. As a result, it is corrupted after
save/restore (replaced by the original value written by L1 on nested
VMRUN). This could cause problems for both KVM (e.g. when injecting a
soft IRQ) or L1 (e.g. when using NextRIP to advance RIP after emulating
an instruction).

Fix this by sync'ing NextRIP to the cache after VMRUN of L2, but only
after completing interrupts (not in nested_sync_control_from_vmcb02()),
as KVM may update NextRIP (e.g. when re-injecting a soft IRQ).

Fixes: cc440cdad5b7 ("KVM: nSVM: implement KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE")
CC: stable@xxxxxxxxxxxxxxx
Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 arch/x86/kvm/svm/svm.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 8f8bc863e2143..07f096758f34f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4435,6 +4435,16 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 
 	svm_complete_interrupts(vcpu);
 
+	/*
+	 * Update the cache after completing interrupts to get an accurate
+	 * NextRIP, e.g. when re-injecting a soft interrupt.
+	 *
+	 * FIXME: Rework svm_get_nested_state() to not pull data from the
+	 *        cache (except for maybe int_ctl).
+	 */
+	if (is_guest_mode(vcpu))
+		svm->nested.ctl.next_rip = svm->vmcb->control.next_rip;
+
 	return svm_exit_handlers_fastpath(vcpu);
 }
 
-- 
2.53.0.414.gf7e9f6c205-goog