[PATCH v2 2/8] KVM: x86/hyperv: Check for NULL vCPU Hyper-V object in kvm_hv_get_tlb_flush_fifo()

[Sean Christopherson <seanjc@google.com>]

Check for a NULL Hyper-V object in kvm_hv_get_tlb_flush_fifo() instead of
relying on the caller to do so.  This will allow fixing a cross-vCPU race
where KVM can access a vCPU's FIFO before it's fully initialized, without
having to jump through too many cognitive hoops to reason about the
correctness of the logic.

Ignoring changes in ordering that only affect the aforementioned race, no
functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/hyperv.c | 11 +++++------
 arch/x86/kvm/hyperv.h |  7 ++++++-
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a894e3d2e594..ecd344b91739 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1939,13 +1939,11 @@ static void hv_tlb_flush_enqueue(struct kvm_vcpu *vcpu, u64 *entries, int count,
 				 bool is_guest_mode)
 {
 	struct kvm_vcpu_hv_tlb_flush_fifo *tlb_flush_fifo;
-	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	u64 flush_all_entry = KVM_HV_TLB_FLUSHALL_ENTRY;
 
-	if (!hv_vcpu)
-		return;
-
 	tlb_flush_fifo = kvm_hv_get_tlb_flush_fifo(vcpu, is_guest_mode);
+	if (!tlb_flush_fifo)
+		return;
 
 	spin_lock(&tlb_flush_fifo->write_lock);
 
@@ -1972,15 +1970,16 @@ static void hv_tlb_flush_enqueue(struct kvm_vcpu *vcpu, u64 *entries, int count,
 int kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
 {
 	struct kvm_vcpu_hv_tlb_flush_fifo *tlb_flush_fifo;
-	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	u64 entries[KVM_HV_TLB_FLUSH_FIFO_SIZE];
 	int i, j, count;
 	gva_t gva;
 
-	if (!tdp_enabled || !hv_vcpu)
+	if (!tdp_enabled)
 		return -EINVAL;
 
 	tlb_flush_fifo = kvm_hv_get_tlb_flush_fifo(vcpu, is_guest_mode(vcpu));
+	if (!tlb_flush_fifo)
+		return -EINVAL;
 
 	count = kfifo_out(&tlb_flush_fifo->entries, entries, KVM_HV_TLB_FLUSH_FIFO_SIZE);
 
diff --git a/arch/x86/kvm/hyperv.h b/arch/x86/kvm/hyperv.h
index 65e89ed65349..e6c74cfbb1cb 100644
--- a/arch/x86/kvm/hyperv.h
+++ b/arch/x86/kvm/hyperv.h
@@ -201,6 +201,9 @@ static inline struct kvm_vcpu_hv_tlb_flush_fifo *kvm_hv_get_tlb_flush_fifo(struc
 	int i = is_guest_mode ? HV_L2_TLB_FLUSH_FIFO :
 				HV_L1_TLB_FLUSH_FIFO;
 
+	if (!hv_vcpu)
+		return NULL;
+
 	return &hv_vcpu->tlb_flush_fifo[i];
 }
 
@@ -208,10 +211,12 @@ static inline void kvm_hv_vcpu_purge_flush_tlb(struct kvm_vcpu *vcpu)
 {
 	struct kvm_vcpu_hv_tlb_flush_fifo *tlb_flush_fifo;
 
-	if (!to_hv_vcpu(vcpu) || !kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
+	if (!kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
 		return;
 
 	tlb_flush_fifo = kvm_hv_get_tlb_flush_fifo(vcpu, is_guest_mode(vcpu));
+	if (!tlb_flush_fifo)
+		return;
 
 	kfifo_reset_out(&tlb_flush_fifo->entries);
 }
-- 
2.54.0.1136.gdb2ca164c4-goog