[PATCH] KVM/Eventfd: Avoid crash when assign and deassign same 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 same 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,
add "initialized" flag in the struct kvm_kernel_irqfd and check the flag before
deactivating irqfd. If irqfd is still in initialization, deassign path
return fault.

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>
Cc: Wanpeng Li <kernel...@gmail.com>
Signed-off-by: Tianyu Lan <tianyu....@intel.com>
---
 include/linux/kvm_irqfd.h |  1 +
 virt/kvm/eventfd.c        | 11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index 76c2fbc..be6b254 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -66,6 +66,7 @@ struct kvm_kernel_irqfd {
        struct work_struct shutdown;
        struct irq_bypass_consumer consumer;
        struct irq_bypass_producer *producer;
+       u8 initialized:1;
 };
 
 #endif /* __LINUX_KVM_IRQFD_H */
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index a334399..80f06e6 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -421,6 +421,7 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
        }
 #endif
 
+       irqfd->initialized = 1;
        return 0;
 
 fail:
@@ -525,6 +526,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
 {
        struct kvm_kernel_irqfd *irqfd, *tmp;
        struct eventfd_ctx *eventfd;
+       int ret = 0;
 
        eventfd = eventfd_ctx_fdget(args->fd);
        if (IS_ERR(eventfd))
@@ -543,7 +545,12 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
                        write_seqcount_begin(&irqfd->irq_entry_sc);
                        irqfd->irq_entry.type = 0;
                        write_seqcount_end(&irqfd->irq_entry_sc);
-                       irqfd_deactivate(irqfd);
+
+                       if (irqfd->initialized)
+                               irqfd_deactivate(irqfd);
+                       else
+                               ret = -EFAULT;
+
                }
        }
 
@@ -557,7 +564,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
         */
        flush_workqueue(irqfd_cleanup_wq);
 
-       return 0;
+       return ret;
 }
 
 int
-- 
1.8.3.1