[PATCH V2] KVM/Eventfd: Avoid crash when assign and deassign specific eventfd in parallel.

[Lan Tianyu]

Syzroot reports crash in kvm_irqfd_assign() is caused by use-after-free.
Because kvm_irqfd_assign() and kvm_irqfd_deassign() can't run in parallel
for one specific eventfd. When assign path hasn't been finished after irqfd
has been added to kvm->irqfds.items list, another thead may deassign the
eventfd and free struct kvm_kernel_irqfd(). This causes assign path still
uses struct kvm_kernel_irqfd freed by deassign path. To avoid such issue,
make irqfd's reference under kvm->irq_srcu protection after irqfd added to
kvm->irqfds.iterms list and call synchronize_srcu() in irq_shutdown() to
make sure that irqfd has been fully initialized in the assign path.

Reported-by: Dmitry Vyukov <dvyu...@google.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Radim Krčmář <rkrc...@redhat.com>
Cc: Dmitry Vyukov <dvyu...@google.com>
Signed-off-by: Tianyu Lan <tianyu....@intel.com>
---
 virt/kvm/eventfd.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index a334399fafec..0abc2e9ddafb 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -119,8 +119,12 @@ irqfd_shutdown(struct work_struct *work)
 {
        struct kvm_kernel_irqfd *irqfd =
                container_of(work, struct kvm_kernel_irqfd, shutdown);
+       struct kvm *kvm = irqfd->kvm;
        u64 cnt;
 
+       /* Make sure irqfd has been initalized in assign path. */
+       synchronize_srcu(&kvm->irq_srcu);
+
        /*
         * Synchronize with the wait-queue and unhook ourselves to prevent
         * further events.
@@ -387,7 +391,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 
        idx = srcu_read_lock(&kvm->irq_srcu);
        irqfd_update(kvm, irqfd);
-       srcu_read_unlock(&kvm->irq_srcu, idx);
 
        list_add_tail(&irqfd->list, &kvm->irqfds.items);
 
@@ -421,6 +424,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
        }
 #endif
 
+       srcu_read_unlock(&kvm->irq_srcu, idx);
        return 0;
 
 fail:
-- 
2.15.1.433.g936d1b9.dirty