[PATCH] TIOCCONS security

The ioctl TIOCCONS allows any user to redirect console output to another
tty.  This allows anyone to suppress messages to the console at will.

AFAIK nowadays not many programs write to /dev/console, except for start
scripts and the kernel (printk() above console log level).

Still, I believe that administrators and operators would not like any user
to be able to hijack messages that were written to the console.

The only user of TIOCCONS that I am aware of is bootlogd/blogd, which runs
as root.  Please comment if there are other users.

Is there any reason why normal users should be able to use TIOCCONS?

Otherwise I would suggest to restrict access to root (CAP_SYS_ADMIN), e.g. 
with this patch.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/tty_io.c

@@ -1981,10 +1981,10 @@ static int tiocswinsz(struct tty_struct *tty, struct tty_struct *real_tty,
 
 static int tioccons(struct file *file)
 {
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
 	if (file->f_op->write == redirected_tty_write) {
 		struct file *f;
-		if (!capable(CAP_SYS_ADMIN))
-			return -EPERM;
 		spin_lock(&redirect_lock);
 		f = redirect;
 		redirect = NULL;