[PATCH] pSeries hvsi char driver null pointer deref

Under certain rare circumstances, it appears that there can be be a
NULL-pointer deref when a user fiddles with terminal emeulation programs while
outpu is being sent to the console.  This patch checks for and avoids a
NULL-pointer deref.

Signed-off-by: Hollis Blanchard <hollisbl@austin.ibm.com>
Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: Paul Fulghum <paulkf@microgate.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/hvsi.c

@@ -311,7 +311,8 @@ static void hvsi_recv_control(struct hvsi_struct *hp, uint8_t *packet,
 				/* CD went away; no more connection */
 				pr_debug("hvsi%i: CD dropped\n", hp->index);
 				hp->mctrl &= TIOCM_CD;
-				if (!(hp->tty->flags & CLOCAL))
+				/* If userland hasn't done an open(2) yet, hp->tty is NULL. */
+				if (hp->tty && !(hp->tty->flags & CLOCAL))
 					*to_hangup = hp->tty;
 			}
 			break;