sch_dsmark: fix potential NULL deref in dsmark_init()

Make sure TCA_DSMARK_INDICES was provided by the user.

syzbot reported :

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8799 Comm: syz-executor235 Not tainted 5.3.0+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:nla_get_u16 include/net/netlink.h:1501 [inline]
RIP: 0010:dsmark_init net/sched/sch_dsmark.c:364 [inline]
RIP: 0010:dsmark_init+0x193/0x640 net/sched/sch_dsmark.c:339
Code: 85 db 58 0f 88 7d 03 00 00 e8 e9 1a ac fb 48 8b 9d 70 ff ff ff 48 b8 00 00 00 00 00 fc ff df 48 8d 7b 04 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 ca
RSP: 0018:ffff88809426f3b8 EFLAGS: 00010247
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff85c6eb09
RDX: 0000000000000000 RSI: ffffffff85c6eb17 RDI: 0000000000000004
RBP: ffff88809426f4b0 R08: ffff88808c4085c0 R09: ffffed1015d26159
R10: ffffed1015d26158 R11: ffff8880ae930ac7 R12: ffff8880a7e96940
R13: dffffc0000000000 R14: ffff88809426f8c0 R15: 0000000000000000
FS:  0000000001292880(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000080 CR3: 000000008ca1b000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 qdisc_create+0x4ee/0x1210 net/sched/sch_api.c:1237
 tc_modify_qdisc+0x524/0x1c50 net/sched/sch_api.c:1653
 rtnetlink_rcv_msg+0x463/0xb00 net/core/rtnetlink.c:5223
 netlink_rcv_skb+0x177/0x450 net/netlink/af_netlink.c:2477
 rtnetlink_rcv+0x1d/0x30 net/core/rtnetlink.c:5241
 netlink_unicast_kernel net/netlink/af_netlink.c:1302 [inline]
 netlink_unicast+0x531/0x710 net/netlink/af_netlink.c:1328
 netlink_sendmsg+0x8a5/0xd60 net/netlink/af_netlink.c:1917
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0xd7/0x130 net/socket.c:657
 ___sys_sendmsg+0x803/0x920 net/socket.c:2311
 __sys_sendmsg+0x105/0x1d0 net/socket.c:2356
 __do_sys_sendmsg net/socket.c:2365 [inline]
 __se_sys_sendmsg net/socket.c:2363 [inline]
 __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2363
 do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440369

Fixes: 758cc43c ("[PKT_SCHED]: Fix dsmark to apply changes consistent")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_dsmark.c

@@ -361,6 +361,8 @@ static int dsmark_init(struct Qdisc *sch, struct nlattr *opt,
 		goto errout;
 
 	err = -EINVAL;
+	if (!tb[TCA_DSMARK_INDICES])
+		goto errout;
 	indices = nla_get_u16(tb[TCA_DSMARK_INDICES]);
 
 	if (hweight32(indices) != 1)