[src/trunk]: src/sys/net Hum. Fix NULL deref triggerable with just write(0).

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/e3bb3f81873c
branches:  trunk
changeset: 935063:e3bb3f81873c
user:      maxv <maxv%NetBSD.org@localhost>
date:      Tue Jun 23 18:30:17 2020 +0000

description:
Hum. Fix NULL deref triggerable with just write(0).

Reported-by: syzbot+45b31355bf880e175b73%syzkaller.appspotmail.com@localhost

diffstat:

 sys/net/if_tun.c |  7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diffs (28 lines):

diff -r 2d34876135c3 -r e3bb3f81873c sys/net/if_tun.c
--- a/sys/net/if_tun.c  Tue Jun 23 17:21:55 2020 +0000
+++ b/sys/net/if_tun.c  Tue Jun 23 18:30:17 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: if_tun.c,v 1.158 2020/01/29 04:34:10 thorpej Exp $     */
+/*     $NetBSD: if_tun.c,v 1.159 2020/06/23 18:30:17 maxv Exp $        */
 
 /*
  * Copyright (c) 1988, Julian Onions <jpo%cs.nott.ac.uk@localhost>
@@ -19,7 +19,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: if_tun.c,v 1.158 2020/01/29 04:34:10 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: if_tun.c,v 1.159 2020/06/23 18:30:17 maxv Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_inet.h"
@@ -857,6 +857,9 @@
 
        TUNDEBUG("%s: tunwrite\n", ifp->if_xname);
 
+       if (uio->uio_resid == 0)
+               return 0;
+
        if (tp->tun_flags & TUN_PREPADDR) {
                if (uio->uio_resid < sizeof(dst)) {
                        error = EIO;