linux-kernel - Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same eventfd in parallel.

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Date:   Mon, 18 Dec 2017 09:30:23 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        kernellwp@...il.com
Subject: Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same
 eventfd in parallel.

On 18.12.2017 00:40, Lan Tianyu wrote:
> 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@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Wanpeng Li <kernellwp@...il.com>
> Signed-off-by: Tianyu Lan <tianyu.lan@...el.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;

The ugly thing in kvm_irqfd_assign() is that we access irqfd without
holding a lock. I think that should rather be fixed than working around
that issue. (e.g. lock() -> lookup again -> verify still in list ->
unlock())

>  	return 0;
>  
>  fail:
> @@ -525,6 +526,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  {

Which tool are you using to generate diffs? git format-patch?

Mentioning, because the indicated function here .... (kvm_irqfd_deassign)

>  	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,
>  	 */

and here are wrong and misleading (kvm_irqfd_deassig). (and just noticed
also in the second hunk)

>  	flush_workqueue(irqfd_cleanup_wq);
>  
> -	return 0;
> +	return ret;
>  }
>  
>  int
> 


-- 

Thanks,

David / dhildenb