net: ipconfig: avoid use-after-free in ic_close_devs

Due to the fact that ic_dev->dev is kept open in ic_close_dev, I had
thought that ic_dev will not be freed either. But that is not the case,
but instead "everybody dies" when ipconfig cleans up, and just the
net_device behind ic_dev->dev remains allocated but not ic_dev itself.

This is a problem because in ic_close_devs, for every net device that
we're about to close, we compare it against the list of lower interfaces
of ic_dev, to figure out whether we should close it or not. But since
ic_dev itself is subject to freeing, this means that at some point in
the middle of the list of ipconfig interfaces, ic_dev will have been
freed, and we would be still attempting to iterate through its list of
lower interfaces while checking whether to bring down the remaining
ipconfig interfaces.

There are multiple ways to avoid the use-after-free: we could delay
freeing ic_dev until the very end (outside the while loop). Or an even
simpler one: we can observe that we don't need ic_dev when iterating
through its lowers, only ic_dev->dev, structure which isn't ever freed.
So, by keeping ic_dev->dev in a variable assigned prior to freeing
ic_dev, we can avoid all use-after-free issues.

Fixes: 46acf7bd

 ("Revert "net: ipv4: handle DSA enabled master network devices"")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>