[PATCH] Fix sysctl unregistration oops (CVE-2005-2709)

You could open the /proc/sys/net/ipv4/conf/<if>/<whatever> file, then
wait for interface to go away, try to grab as much memory as possible in
hope to hit the (kfreed) ctl_table.  Then fill it with pointers to your
function.  Then do read from file you've opened and if you are lucky,
you'll get it called as ->proc_handler() in kernel mode.

So this is at least an Oops and possibly more.  It does depend on an
interface going away though, so less of a security risk than it would
otherwise be.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sysctl.h

@@ -24,6 +24,7 @@
 #include <linux/compiler.h>
 
 struct file;
+struct completion;
 
 #define CTL_MAXNAME 10		/* how many path components do we allow in a
 				   call to sysctl?   In other words, what is
@@ -925,6 +926,8 @@ struct ctl_table_header
 {
 	ctl_table *ctl_table;
 	struct list_head ctl_entry;
+	int used;
+	struct completion *unregistering;
 };
 
 struct ctl_table_header * register_sysctl_table(ctl_table * table,