sch_choke: avoid potential panic in choke_reset()

If choke_init() could not allocate q->tab, we would crash later
in choke_reset().

BUG: KASAN: null-ptr-deref in memset include/linux/string.h:366 [inline]
BUG: KASAN: null-ptr-deref in choke_reset+0x208/0x340 net/sched/sch_choke.c:326
Write of size 8 at addr 0000000000000000 by task syz-executor822/7022

CPU: 1 PID: 7022 Comm: syz-executor822 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 __kasan_report.cold+0x5/0x4d mm/kasan/report.c:515
 kasan_report+0x33/0x50 mm/kasan/common.c:625
 check_memory_region_inline mm/kasan/generic.c:187 [inline]
 check_memory_region+0x141/0x190 mm/kasan/generic.c:193
 memset+0x20/0x40 mm/kasan/common.c:85
 memset include/linux/string.h:366 [inline]
 choke_reset+0x208/0x340 net/sched/sch_choke.c:326
 qdisc_reset+0x6b/0x520 net/sched/sch_generic.c:910
 dev_deactivate_queue.constprop.0+0x13c/0x240 net/sched/sch_generic.c:1138
 netdev_for_each_tx_queue include/linux/netdevice.h:2197 [inline]
 dev_deactivate_many+0xe2/0xba0 net/sched/sch_generic.c:1195
 dev_deactivate+0xf8/0x1c0 net/sched/sch_generic.c:1233
 qdisc_graft+0xd25/0x1120 net/sched/sch_api.c:1051
 tc_modify_qdisc+0xbab/0x1a00 net/sched/sch_api.c:1670
 rtnetlink_rcv_msg+0x44e/0xad0 net/core/rtnetlink.c:5454
 netlink_rcv_skb+0x15a/0x410 net/netlink/af_netlink.c:2469
 netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
 netlink_unicast+0x537/0x740 net/netlink/af_netlink.c:1329
 netlink_sendmsg+0x882/0xe10 net/netlink/af_netlink.c:1918
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:672
 ____sys_sendmsg+0x6bf/0x7e0 net/socket.c:2362
 ___sys_sendmsg+0x100/0x170 net/socket.c:2416
 __sys_sendmsg+0xec/0x1b0 net/socket.c:2449
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295

Fixes: 77e62da6 ("sch_choke: drop all packets in queue during reset")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_choke.c

@@ -323,7 +323,8 @@ static void choke_reset(struct Qdisc *sch)
 
 	sch->q.qlen = 0;
 	sch->qstats.backlog = 0;
-	memset(q->tab, 0, (q->tab_mask + 1) * sizeof(struct sk_buff *));
+	if (q->tab)
+		memset(q->tab, 0, (q->tab_mask + 1) * sizeof(struct sk_buff *));
 	q->head = q->tail = 0;
 	red_restart(&q->vars);
 }