[PATCH] KVM: arm64: Hold kvm->mmu_lock while initialising vcpu->arch.vncr_tlb

[Marc Zyngier]

Sashiko reports that there is a race between initialising vncr_tlb
and making use of it, as we don't hold the mmu_lock at this point.

Additionally, it identifies a memory leak, should userspace repeatedly
invokes the KVM_RUN ioctl after a failure of kvm_arch_vcpu_run_pid_change(),
as we assign vncr_tlb blindly on first run, irrespective of prior
allocations.

Slap the two bugs in one go by taking the kvm->mmu_lock on assigning
vncr_tlb, preventing the race for good, and by checking that vncr_tlb
is indeed NULL prior to allocation.

Reported-by: Sashiko <sashiko-bot at kernel.org>
Signed-off-by: Marc Zyngier <maz at kernel.org>
Link: https://lore.kernel.org/r/20260607180815.85FBC1F00893@smtp.kernel.org
---
 arch/arm64/kvm/nested.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 690b8e8564166..d11e36b3cfcc2 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1253,8 +1253,14 @@ int kvm_vcpu_allocate_vncr_tlb(struct kvm_vcpu *vcpu)
 	if (!kvm_has_feat(vcpu->kvm, ID_AA64MMFR4_EL1, NV_frac, NV2_ONLY))
 		return 0;
 
-	vcpu->arch.vncr_tlb = kzalloc_obj(*vcpu->arch.vncr_tlb,
-					  GFP_KERNEL_ACCOUNT);
+	if (!vcpu->arch.vncr_tlb) {
+		struct vncr_tlb *vt = kzalloc_obj(*vcpu->arch.vncr_tlb,
+						  GFP_KERNEL_ACCOUNT);
+
+		scoped_guard(write_lock, &vcpu->kvm->mmu_lock)
+			vcpu->arch.vncr_tlb = vt;
+	}
+
 	if (!vcpu->arch.vncr_tlb)
 		return -ENOMEM;
 
-- 
2.47.3