Re: Kernel panic with vhost-vdpa

On 2021/2/18 11:15 下午, Gautam Dawar wrote:

Hi Jason,

Thanks for your response.

On Thu, 18 Feb 2021 at 14:18, Jason Wang [off-list ref] wrote:

quoted

Hi Gautam:
On 2021/2/15 9:01 下午, Gautam Dawar wrote:

Hi Jason/Michael,



I observed a kernel panic while testing vhost-vdpa with Xilinx adapters.
Here are the details for your review:



Problem statement:

When qemu with vhost-vdpa netdevice is run for the first time, it works
well. But after the VM is powered off, next qemu run causes kernel panic
due to a NULL pointer dereference in irq_bypass_register_producer().



Root cause analysis:

When the VM is powered off, vhost_dev_stop() is invoked which in turn
calls vhost_vdpa_reset_device() causing the irq_bypass producers to be
unregistered.



On the next run, when qemu opens the vhost device, the vhost_vdpa_open()
file operation calls vhost_dev_init(). Here, call_ctx->producer memory is
cleared in vhost_vring_call_reset().



Further, when the virtqueues are initialized by vhost_virtqueue_init(),
vhost_vdpa_setup_vq_irq() again registers the irq_bypass producer for each
virtqueue. As the node member of struct irq_bypass_producer is also
initialized to zero, traversal on the producers list causes crash due to
NULL pointer dereference.


Thanks a lot for reporting this issue.




Fix details:



I think that this issue can be fixed by invoking
vhost_vdpa_setup_vq_irq() only when vhost_vdpa_set_status() includes
VIRTIO_CONFIG_S_DRIVER_OK in the new status value. This way, there won’t be
any stale nodes in the irqbypass  module’s producers list which are reset
in vhost_vring_call_reset().



Patch:

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index

62a9bb0efc55..fdad94e2fbf9 100644

--- a/drivers/vhost/vdpa.c

+++ b/drivers/vhost/vdpa.c

@@ -409,7 +409,6 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa

*v, unsigned int cmd,

                        cb.private = NULL;

                }

                ops->set_vq_cb(vdpa, idx, &cb);

-               vhost_vdpa_setup_vq_irq(v, idx);

                break;



        case VHOST_SET_VRING_NUM:



We can also track this issue in Bugzilla ticket 21171 (
https://bugzilla.kernel.org/show_bug.cgi?id=211711)  and the complete
patch is attached with this email.


So vhost supports to remove or switch eventfd through
vhost_vdpa_vring_ioctl(). So if userspace want to switch to another
eventfd, we should re-do the register and unregister.

GD>>  This makes sense. I missed the use case where userspace may want to
switch to a different eventfd.


This can happen when interrupt needs to be disabled for some reason (e.g
MSI-X is masked).



I think we need to deal this issue in another way. Can we check whether or

quoted

not the producer is initialized before?

Thanks

GD>> Initialization path is fine but the actual problem lies in the
clean-up part.
I think the following check is the cause of this issue:

static void vhost_vdpa_clean_irq(struct vhost_vdpa *v)
                if (vq->call_ctx.producer.irq)

irq_bypass_unregister_producer(&vq->call_ctx.producer);

The above if condition will prevent the de-initialization of the producer
nodes corresponding to irq 0 but  irq_bypass_unregister_producer() should
be called for all valid irq values including zero.

Accordingly, following patch is required to fix this issue:

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 62a9bb0efc55..d1c3a33c6239 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c

@@ -849,7 +849,7 @@ static void vhost_vdpa_clean_irq(struct vhost_vdpa *v)

        for (i = 0; i < v->nvqs; i++) {
                vq = &v->vqs[i];
-               if (vq->call_ctx.producer.irq)
+               if (vq->call_ctx.producer.irq >= 0)

irq_bypass_unregister_producer(&vq->call_ctx.producer);
        }
 }


It should work, please post a formal patch for this.

I will give more thought in the meanwhile since I spot some other defects
on codes for irqbyass usage in vdpa.

Thanks



The revised patch (bug211711_fix.patch) is also attached with this email.

quoted

Regards,

Gautam Dawar