Patch "KVM: arm64: Destroy mpidr_data for 'late' vCPU creation" has been added to the 6.8-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: arm64: Destroy mpidr_data for 'late' vCPU creation

to the 6.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-arm64-destroy-mpidr_data-for-late-vcpu-creation.patch
and it can be found in the queue-6.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 7b0f4f9b38735149a0c7ce134fbcc073f73962c1
Author: Oliver Upton <oliver.upton@xxxxxxxxx>
Date:   Wed May 8 07:19:52 2024 +0000

    KVM: arm64: Destroy mpidr_data for 'late' vCPU creation
    
    [ Upstream commit ce5d2448eb8fe83aed331db53a08612286a137dd ]
    
    A particularly annoying userspace could create a vCPU after KVM has
    computed mpidr_data for the VM, either by racing against VGIC
    initialization or having a userspace irqchip.
    
    In any case, this means mpidr_data no longer fully describes the VM, and
    attempts to find the new vCPU with kvm_mpidr_to_vcpu() will fail. The
    fix is to discard mpidr_data altogether, as it is only a performance
    optimization and not required for correctness. In all likelihood KVM
    will recompute the mappings when KVM_RUN is called on the new vCPU.
    
    Note that reads of mpidr_data are not guarded by a lock; promote to RCU
    to cope with the possibility of mpidr_data being invalidated at runtime.
    
    Fixes: 54a8006d0b49 ("KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is available")
    Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
    Link: https://lore.kernel.org/r/20240508071952.2035422-1-oliver.upton@xxxxxxxxx
    Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index c063e84fc72c7..0f340143a1e74 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -191,6 +191,23 @@ vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
 }
 
 
+static void kvm_destroy_mpidr_data(struct kvm *kvm)
+{
+	struct kvm_mpidr_data *data;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	data = rcu_dereference_protected(kvm->arch.mpidr_data,
+					 lockdep_is_held(&kvm->arch.config_lock));
+	if (data) {
+		rcu_assign_pointer(kvm->arch.mpidr_data, NULL);
+		synchronize_rcu();
+		kfree(data);
+	}
+
+	mutex_unlock(&kvm->arch.config_lock);
+}
+
 /**
  * kvm_arch_destroy_vm - destroy the VM data structure
  * @kvm:	pointer to the KVM struct
@@ -205,7 +222,8 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	if (is_protected_kvm_enabled())
 		pkvm_destroy_hyp_vm(kvm);
 
-	kfree(kvm->arch.mpidr_data);
+	kvm_destroy_mpidr_data(kvm);
+
 	kfree(kvm->arch.sysreg_masks);
 	kvm_destroy_vcpus(kvm);
 
@@ -391,6 +409,13 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
 
+	/*
+	 * This vCPU may have been created after mpidr_data was initialized.
+	 * Throw out the pre-computed mappings if that is the case which forces
+	 * KVM to fall back to iteratively searching the vCPUs.
+	 */
+	kvm_destroy_mpidr_data(vcpu->kvm);
+
 	err = kvm_vgic_vcpu_init(vcpu);
 	if (err)
 		return err;
@@ -590,7 +615,8 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 
 	mutex_lock(&kvm->arch.config_lock);
 
-	if (kvm->arch.mpidr_data || atomic_read(&kvm->online_vcpus) == 1)
+	if (rcu_access_pointer(kvm->arch.mpidr_data) ||
+	    atomic_read(&kvm->online_vcpus) == 1)
 		goto out;
 
 	kvm_for_each_vcpu(c, vcpu, kvm) {
@@ -627,7 +653,7 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 		data->cmpidr_to_idx[index] = c;
 	}
 
-	kvm->arch.mpidr_data = data;
+	rcu_assign_pointer(kvm->arch.mpidr_data, data);
 out:
 	mutex_unlock(&kvm->arch.config_lock);
 }
@@ -2460,21 +2486,27 @@ static int __init init_hyp_mode(void)
 
 struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr)
 {
-	struct kvm_vcpu *vcpu;
+	struct kvm_vcpu *vcpu = NULL;
+	struct kvm_mpidr_data *data;
 	unsigned long i;
 
 	mpidr &= MPIDR_HWID_BITMASK;
 
-	if (kvm->arch.mpidr_data) {
-		u16 idx = kvm_mpidr_index(kvm->arch.mpidr_data, mpidr);
+	rcu_read_lock();
+	data = rcu_dereference(kvm->arch.mpidr_data);
 
-		vcpu = kvm_get_vcpu(kvm,
-				    kvm->arch.mpidr_data->cmpidr_to_idx[idx]);
+	if (data) {
+		u16 idx = kvm_mpidr_index(data, mpidr);
+
+		vcpu = kvm_get_vcpu(kvm, data->cmpidr_to_idx[idx]);
 		if (mpidr != kvm_vcpu_get_mpidr_aff(vcpu))
 			vcpu = NULL;
+	}
 
+	rcu_read_unlock();
+
+	if (vcpu)
 		return vcpu;
-	}
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (mpidr == kvm_vcpu_get_mpidr_aff(vcpu))