Char: lp, protect LPGETSTATUS with port_mutex

The patch fixes a problem in the lp driver that can cause oopses as
follows:
process A:	calls lp_write, which in turn calls
		parport_ieee1284_write_compat, and that invokes
		parport_wait_peripheral
process B:	meanwhile does an ioctl(LPGETSTATUS), which call
		lp_release_parport when done. This function will set
		physport->cad = NULL.
process A:	parport_wait_peripheral tries to dereference
		physport->cad and dies

So, protect that code with the port_mutex in order to protect against
simultaneous calls to lp_read/lp_write.

Similar protection is probably required for ioctl(LPRESET)...

This patch was done by IBM a while back and we (at suse) have that
since at least 2004 in our repos. Let's make it upstream.

Signed-off-by: okir@suse.de
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/lp.c

@@ -622,9 +622,12 @@ static int lp_do_ioctl(unsigned int minor, unsigned int cmd,
 				return -EFAULT;
 			break;
 		case LPGETSTATUS:
+			if (mutex_lock_interruptible(&lp_table[minor].port_mutex))
+				return -EINTR;
 			lp_claim_parport_or_block (&lp_table[minor]);
 			status = r_str(minor);
 			lp_release_parport (&lp_table[minor]);
+			mutex_unlock(&lp_table[minor].port_mutex);
 
 			if (copy_to_user(argp, &status, sizeof(int)))
 				return -EFAULT;