UBUNTU: SAUCE: apparmor: Don't audit denied access of special apparmor .null file

When an fd is disallowed from being inherited during exec, instead of
closed it is duped to a special apparmor/.null file. This prevents the
fd from being reused by another file in case the application expects
the original file on a give fd (eg stdin/stdout etc). This results in
a denial message like
[32375.561535] audit: type=1400 audit(1478825963.441:358): apparmor="DENIED" operation="file_inherit" namespace="root//lxd-t_<var-lib-lxd>" profile="/sbin/dhclient" name="/dev/pts/1" pid=16795 comm="dhclient" requested_mask="wr" denied_mask="wr" fsuid=165536 ouid=165536

Further access to the fd is resultin in the rather useless denial message
of
[32375.566820] audit: type=1400 audit(1478825963.445:359): apparmor="DENIED" operation="file_perm" namespace="root//lxd-t_<var-lib-lxd>" profile="/sbin/dhclient" name="/apparmor/.null" pid=16795 comm="dhclient" requested_mask="w" denied_mask="w" fsuid=165536 ouid=0

since we have the original denial, the noisy and useless .null based
denials can be skipped.

BugLink: http://bugs.launchpad.net/bugs/1660836Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

security/apparmor/lsm.c

@@ -443,6 +443,9 @@ static int common_file_perm(const char *op, struct file *file, u32 mask)
 	struct aa_label *label;
 	int error = 0;
 
+	if (file->f_path.dentry == aa_null.dentry)
+		return -EACCES;
+
 	label = aa_begin_current_label(NO_UPDATE);
 	error = aa_file_perm(op, label, file, mask);
 	aa_end_current_label(label);