[PATCH 4/6] Revert "KVM: Fix vcpu_array[0] races"

Sean Christopherson
2024-10-09T15:05:06Z

Now that KVM loads from vcpu_array if and only if the target index is
valid with respect to online_vcpus, i.e. now that it is safe to erase a
not-fully-onlined vCPU entry, revert to storing into vcpu_array before
success is guaranteed.

If xa_store() fails, which _should_ be impossible, then putting the vCPU's
reference to 'struct kvm' results in a refcounting bug as the vCPU fd has
been installed and owns the vCPU's reference.

This was found by inspection, but forcing the xa_store() to fail
confirms the problem:

| Unable to handle kernel paging request at virtual address ffff800080ecd960
| Call trace:
| _raw_spin_lock_irq+0x2c/0x70
| kvm_irqfd_release+0x24/0xa0
| kvm_vm_release+0x1c/0x38
| __fput+0x88/0x2ec
| ____fput+0x10/0x1c
| task_work_run+0xb0/0xd4
| do_exit+0x210/0x854
| do_group_exit+0x70/0x98
| get_signal+0x6b0/0x73c
| do_signal+0xa4/0x11e8
| do_notify_resume+0x60/0x12c
| el0_svc+0x64/0x68
| el0t_64_sync_handler+0x84/0xfc
| el0t_64_sync+0x190/0x194
| Code: b9000909 d503201f 2a1f03e1 52800028 (88e17c08)

Practically speaking, this is a non-issue as xa_store() can't fail, absent
a nasty kernel bug. But the code is visually jarring and technically
broken.

This reverts commit afb2acb2e3a32e4d56f7fbd819769b98ed1b7520.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michal Luczaj <mhal@rbox.co>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
virt/kvm/kvm_main.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fca9f74e9544..f081839521ef 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4283,7 +4283,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
}

vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
- r = xa_reserve(&kvm->vcpu_array, vcpu->vcpu_idx, GFP_KERNEL_ACCOUNT);
+ r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
+ BUG_ON(r == -EBUSY);
if (r)
goto unlock_vcpu_destroy;

@@ -4298,12 +4299,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
kvm_get_kvm(kvm);
r = create_vcpu_fd(vcpu);
if (r < 0)
- goto kvm_put_xa_release;
-
- if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
- r = -EINVAL;
- goto kvm_put_xa_release;
- }
+ goto kvm_put_xa_erase;

/*
* Pairs with smp_rmb() in kvm_get_vcpu. Store the vcpu
@@ -4318,10 +4314,10 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
kvm_create_vcpu_debugfs(vcpu);
return r;

-kvm_put_xa_release:
+kvm_put_xa_erase:
mutex_unlock(&vcpu->mutex);
kvm_put_kvm_no_destroy(kvm);
- xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
+ xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx);
unlock_vcpu_destroy:
mutex_unlock(&kvm->lock);
kvm_dirty_ring_free(&vcpu->dirty_ring);
--
2.47.0.rc0.187.ge670bccf7e-goog