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Subject[PATCH] KVM/Eventfd: Avoid crash when assign and deassign same eventfd in parallel.
Date
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 <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Wanpeng Li <kernellwp@gmail.com>
Signed-off-by: Tianyu Lan <tianyu.lan@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
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