proc: prevent changes to overridden credentials

Prevent userspace from changing the the /proc/PID/attr values if the
task's credentials are currently overriden.  This not only makes sense
conceptually, it also prevents some really bizarre error cases caused
when trying to commit credentials to a task with overridden
credentials.

Cc: <stable@vger.kernel.org>
Reported-by: N"chengjian (D)" <cj.chengjian@huawei.com>
Signed-off-by: NPaul Moore <paul@paul-moore.com>
Acked-by: NJohn Johansen <john.johansen@canonical.com>
Acked-by: NJames Morris <james.morris@microsoft.com>
Acked-by: NCasey Schaufler <casey@schaufler-ca.com>

fs/proc/base.c

@@ -2539,6 +2539,11 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 		rcu_read_unlock();
 		return -EACCES;
 	}
+	/* Prevent changes to overridden credentials. */
+	if (current_cred() != current_real_cred()) {
+		rcu_read_unlock();
+		return -EBUSY;
+	}
 	rcu_read_unlock();
 
 	if (count > PAGE_SIZE)