[PATCH 4.14 201/217] KVM: irqfd: fix race between EPOLLHUP and irq_bypass_register_consumer

Greg Kroah-Hartman
2018-08-23T08:38:59Z

From: Paolo Bonzini <pbonzini@redhat.com>

commit 9432a3175770e06cb83eada2d91fac90c977cb99 upstream.

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@googlegroups.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
virt/kvm/eventfd.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -405,11 +405,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct
if (events & POLLIN)
schedule_work(&irqfd->inject);

- /*
- * do not drop the file until the irqfd is fully initialized, otherwise
- * we might race against the POLLHUP
- */
- fdput(f);
#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
if (kvm_arch_has_irq_bypass()) {
irqfd->consumer.token = (void *)irqfd->eventfd;
@@ -425,6 +420,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct
#endif

srcu_read_unlock(&kvm->irq_srcu, idx);
+
+ /*
+ * do not drop the file until the irqfd is fully initialized, otherwise
+ * we might race against the POLLHUP
+ */
+ fdput(f);
return 0;