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Subject: Re: [PATCH V2 06/12] virtio_pci: harden MSI-X interrupts
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>, david.kaplan@amd.com,
        konrad.wilk@oracle.com, Peter Zijlstra <peterz@infradead.org>,
        f.hetzelt@tu-berlin.de, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Thomas Gleixner <tglx@linutronix.de>, mst@redhat.com
References: <20211012065227.9953-1-jasowang@redhat.com>
 <20211012065227.9953-7-jasowang@redhat.com>
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <c6641b6a-6204-2b41-e775-ad329314711c@oracle.com>
Date:   Fri, 15 Oct 2021 05:09:38 -0700
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Hi Jason,

On 10/11/21 11:52 PM, Jason Wang wrote:
> We used to synchronize pending MSI-X irq handlers via
> synchronize_irq(), this may not work for the untrusted device which
> may keep sending interrupts after reset which may lead unexpected
> results. Similarly, we should not enable MSI-X interrupt until the

About "unexpected results", while you mentioned below in v1 ...

"My understanding is that e.g in the case of SEV/TDX we don't trust the
hypervisor. So the hypervisor can keep sending interrupts even if the
device is reset. The guest can only trust its own software interrupt
management logic to avoid call virtio callback in this case."

.. and you also mentioned to avoid the panic (due to untrusted device) in as
many scenarios as possible.


Here is my understanding.

The reason we do not trust hypervisor is to protect (1) data/code privacy for
most of times and sometimes (2) program execution integrity.

The bad thing is: the hypervisor is able to panic/destroy the VM in the worst case.

It is reasonable to re-configure/recover if we assume there is BUG at
hypervisor/device side. That is, the hypervisor/device is buggy, but not malicious.

However, how about to just detect and report the hypervisor/device is malicious
and shutdown/panic the VM immediately? If we have detected the malicious
hypervisor, we should avoid running VMs on the malicious hypervisor further. At
least how about to print error message to reminder users that the hypervisor is
malicious?


About "unexpected results", it should not be hang/panic. I suggest ...

Assuming SEV/TDX is involved, the hypervisor should never be able to derive the
VM privacy (at least secure memory data) by providing malicious configuration,
e.g., num_queues=0. If we detect hypervisor is malicious, the VM is
panic/shutdown immediately. At least how about to print error message to
reminder users.


BTW, while I always do care about the loss of interrupt issue, the malicious
device is able to hang a VM by dropping a single interrupt on purpose for
virtio-scsi :)


Thank you very much!

Dongli Zhang

> device is ready. So this patch fixes those two issues by:
> 
> 1) switching to use disable_irq() to prevent the virtio interrupt
>    handlers to be called after the device is reset.
> 2) using IRQF_NO_AUTOEN and enable the MSI-X irq during .ready()
> 
> This can make sure the virtio interrupt handler won't be called before
> virtio_device_ready() and after reset.
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_pci_common.c | 27 +++++++++++++++++++++------
>  drivers/virtio/virtio_pci_common.h |  6 ++++--
>  drivers/virtio/virtio_pci_legacy.c |  5 +++--
>  drivers/virtio/virtio_pci_modern.c |  6 ++++--
>  4 files changed, 32 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index b35bb2d57f62..0b9523e6dd39 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -24,8 +24,8 @@ MODULE_PARM_DESC(force_legacy,
>  		 "Force legacy mode for transitional virtio 1 devices");
>  #endif
>  
> -/* wait for pending irq handlers */
> -void vp_synchronize_vectors(struct virtio_device *vdev)
> +/* disable irq handlers */
> +void vp_disable_vectors(struct virtio_device *vdev)
>  {
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	int i;
> @@ -34,7 +34,20 @@ void vp_synchronize_vectors(struct virtio_device *vdev)
>  		synchronize_irq(vp_dev->pci_dev->irq);
>  
>  	for (i = 0; i < vp_dev->msix_vectors; ++i)
> -		synchronize_irq(pci_irq_vector(vp_dev->pci_dev, i));
> +		disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> +}
> +
> +/* enable irq handlers */
> +void vp_enable_vectors(struct virtio_device *vdev)
> +{
> +	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> +	int i;
> +
> +	if (vp_dev->intx_enabled)
> +		return;
> +
> +	for (i = 0; i < vp_dev->msix_vectors; ++i)
> +		enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
>  }
>  
>  /* the notify function used when creating a virt queue */
> @@ -141,7 +154,8 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  	snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
>  		 "%s-config", name);
>  	err = request_irq(pci_irq_vector(vp_dev->pci_dev, v),
> -			  vp_config_changed, 0, vp_dev->msix_names[v],
> +			  vp_config_changed, IRQF_NO_AUTOEN,
> +			  vp_dev->msix_names[v],
>  			  vp_dev);
>  	if (err)
>  		goto error;
> @@ -160,7 +174,8 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  		snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
>  			 "%s-virtqueues", name);
>  		err = request_irq(pci_irq_vector(vp_dev->pci_dev, v),
> -				  vp_vring_interrupt, 0, vp_dev->msix_names[v],
> +				  vp_vring_interrupt, IRQF_NO_AUTOEN,
> +				  vp_dev->msix_names[v],
>  				  vp_dev);
>  		if (err)
>  			goto error;
> @@ -337,7 +352,7 @@ static int vp_find_vqs_msix(struct virtio_device *vdev, unsigned nvqs,
>  			 "%s-%s",
>  			 dev_name(&vp_dev->vdev.dev), names[i]);
>  		err = request_irq(pci_irq_vector(vp_dev->pci_dev, msix_vec),
> -				  vring_interrupt, 0,
> +				  vring_interrupt, IRQF_NO_AUTOEN,
>  				  vp_dev->msix_names[msix_vec],
>  				  vqs[i]);
>  		if (err)
> diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> index beec047a8f8d..a235ce9ff6a5 100644
> --- a/drivers/virtio/virtio_pci_common.h
> +++ b/drivers/virtio/virtio_pci_common.h
> @@ -102,8 +102,10 @@ static struct virtio_pci_device *to_vp_device(struct virtio_device *vdev)
>  	return container_of(vdev, struct virtio_pci_device, vdev);
>  }
>  
> -/* wait for pending irq handlers */
> -void vp_synchronize_vectors(struct virtio_device *vdev);
> +/* disable irq handlers */
> +void vp_disable_vectors(struct virtio_device *vdev);
> +/* enable irq handlers */
> +void vp_enable_vectors(struct virtio_device *vdev);
>  /* the notify function used when creating a virt queue */
>  bool vp_notify(struct virtqueue *vq);
>  /* the config->del_vqs() implementation */
> diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
> index d62e9835aeec..bdf6bc667ab5 100644
> --- a/drivers/virtio/virtio_pci_legacy.c
> +++ b/drivers/virtio/virtio_pci_legacy.c
> @@ -97,8 +97,8 @@ static void vp_reset(struct virtio_device *vdev)
>  	/* Flush out the status write, and flush in device writes,
>  	 * including MSi-X interrupts, if any. */
>  	ioread8(vp_dev->ioaddr + VIRTIO_PCI_STATUS);
> -	/* Flush pending VQ/configuration callbacks. */
> -	vp_synchronize_vectors(vdev);
> +	/* Disable VQ/configuration callbacks. */
> +	vp_disable_vectors(vdev);
>  }
>  
>  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> @@ -194,6 +194,7 @@ static void del_vq(struct virtio_pci_vq_info *info)
>  }
>  
>  static const struct virtio_config_ops virtio_pci_config_ops = {
> +	.ready		= vp_enable_vectors,
>  	.get		= vp_get,
>  	.set		= vp_set,
>  	.get_status	= vp_get_status,
> diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> index 30654d3a0b41..acf0f6b6381d 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -172,8 +172,8 @@ static void vp_reset(struct virtio_device *vdev)
>  	 */
>  	while (vp_modern_get_status(mdev))
>  		msleep(1);
> -	/* Flush pending VQ/configuration callbacks. */
> -	vp_synchronize_vectors(vdev);
> +	/* Disable VQ/configuration callbacks. */
> +	vp_disable_vectors(vdev);
>  }
>  
>  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> @@ -380,6 +380,7 @@ static bool vp_get_shm_region(struct virtio_device *vdev,
>  }
>  
>  static const struct virtio_config_ops virtio_pci_config_nodev_ops = {
> +	.ready		= vp_enable_vectors,
>  	.get		= NULL,
>  	.set		= NULL,
>  	.generation	= vp_generation,
> @@ -397,6 +398,7 @@ static const struct virtio_config_ops virtio_pci_config_nodev_ops = {
>  };
>  
>  static const struct virtio_config_ops virtio_pci_config_ops = {
> +	.ready		= vp_enable_vectors,
>  	.get		= vp_get,
>  	.set		= vp_set,
>  	.generation	= vp_generation,
>