Running VirtualBox on Debian needs the virtualbox-dkms package installed. DKMS stands for Dynamic Kernel Module Support and is an attempt to build out-of-tree drivers for the kernel versions installed instead of offering multiple package versions of the same driver.

So, virtualbox-dkms was installed and all was good - until a change in the kernel sources broke the VirtualBox build and needed a patch against the virtualbox-dkms sources. However, only recent kernel versions were affected, the patched virtualbox-dkms code would not run correctly with an older kernel.

On this box, two kernel versions are installed: linux-image-3.14-2-amd64 and linux-image-3.17.0-rc1+, compiled from vanilla sources. A "dpkg-reconfigure virtualbox-dkms" would build the virtualbox-dkms for both kernel versions, but for the reasons explained above, we can't do this now.

Let's rebuild virtualbox-dkms only for the kernel that needs to be built with the patched version of virtualbox-dkms:

# rmmod vboxpci vboxnetadp vboxnetflt vboxdrv

# ls -lgo /var/lib/dkms/virtualbox/
total 4
drwxr-xr-x 5 4096 Aug 31 01:49 4.3.14
lrwxrwxrwx 1   26 Aug 31 01:49 kernel-3.14-2-amd64-x86_64 -> 4.3.14/3.14-2-amd64/x86_64
lrwxrwxrwx 1   25 Aug 31 01:37 kernel-3.17.0-rc1+-x86_64 -> 4.3.14/3.17.0-rc1+/x86_64

# dkms remove virtualbox/4.3.14 -k 3.17.0-rc1+/x86_64

# cd /usr/src/virtualbox-4.3.14
# patch -p0 < ~/virtualbox-alloc_netdev.diff
# dkms install virtualbox/4.3.14 -k 3.17.0-rc1+/x86_64

And that should be all to it :)