Re: Kernel panic with vhost-vdpa

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.