security/tomoyo: Add a special case to handle accesses through the internal proc mount.

With the change of sys_sysctl going through the internal proc mount we no
longer need to handle security_sysctl in tomoyo as we have valid pathnames
for all sysctl accesses.  There is one slight caveat to that in that
all of the paths from the internal mount look like
"/sys/net/ipv4/ip_local_port_range" instead of
"/proc/sys/net/ipv4/ip_local_port_range" so tomoyo needs to add the
"/proc" portion manually when resolving to full path names to get what it expects.

This change teaches tomoyo perform that modification.

Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

security/tomoyo/realpath.c

@@ -108,6 +108,15 @@ int tomoyo_realpath_from_path2(struct path *path, char *newname,
 		spin_unlock(&dcache_lock);
 		path_put(&root);
 		path_put(&ns_root);
+		/* Prepend "/proc" prefix if using internal proc vfs mount. */
+		if (!IS_ERR(sp) && (path->mnt->mnt_parent == path->mnt) &&
+		    (strcmp(path->mnt->mnt_sb->s_type->name, "proc") == 0)) {
+			sp -= 5;
+			if (sp >= newname)
+				memcpy(sp, "/proc", 5);
+			else
+				sp = ERR_PTR(-ENOMEM);
+		}
 	}
 	if (IS_ERR(sp))
 		error = PTR_ERR(sp);