Bug 30630

if_mib.c:sysctl_ifdata fails to check whether the accessed interface really
exists.

The problem is, that since the advent of loadable modules the interface
index name space may be sparse. Before loadable modules one was sure that
all interfaces between 1 and if_index really exists so the check

   if (name[0] <= 0 || name[0] > if_index)

was ok. Now it is possible to unload interface drivers so that interfaces
between 1 and if_index may disappear. ifaddr_byindex(IDX) will return NULL
in this case which in turn leads to a kernel panic.

There may be other places in the kernel that also build on the old assumption.

How-To-Repeat: 
Put 2 network cards in your computer which need different drivers. Build
these drivers as loadable modules and reboot. Now configure the two interfaces.
Now unload the driver for the first interface and execute the test program
below:

# include "stdio.h"
# include "err.h"
# include "sys/types.h"
# include "sys/sysctl.h"
# include "sys/socket.h"
# include "net/if.h"
# include "net/if_mib.h"

int
main(int argc, char *argv[])
{
	int name[6];
	size_t len;

	name[0] = CTL_NET;
	name[1] = PF_LINK;
	name[2] = NETLINK_GENERIC;
	name[3] = IFMIB_IFDATA;
	name[4] = 4;
	name[5] = IFDATA_LINKSPECIFIC;

	if (sysctl(name, 6, NULL, &len, NULL, 0) < 0) {
		err(1, "sysctl failed");
		return (1);
	}
	return (0);
}

Watch the kernel panic.