A comment warning against this bug is there, but the code is not doing what
the comment says. Therefore it is possible that an EPOLLHUP races against
irq_bypass_register_consumer. The EPOLLHUP handler schedules irqfd_shutdown,
and if that runs soon enough, you get a use-after-free.
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
virt/kvm/eventfd.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 6e865e8b5b10..44dda5dad0ee 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -402,11 +402,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
if (events & EPOLLIN)
schedule_work(&irqfd->inject);
- /*
- * do not drop the file until the irqfd is fully initialized, otherwise
- * we might race against the EPOLLHUP
- */
- fdput(f);
#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
if (kvm_arch_has_irq_bypass()) {
irqfd->consumer.token = (void *)irqfd->eventfd;
@@ -421,6 +416,11 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
}
#endif
+ /*
+ * do not drop the file until the irqfd is fully initialized, otherwise
+ * we might race against the EPOLLHUP
+ */
+ fdput(f);
return 0;
fail:
--
2.17.0
