net/dccp: fix use after free in tw_timer_handler()

DCCP doesn't purge timewait sockets on network namespace shutdown.
So, after net namespace destroyed we could still have an active timer
which will trigger use after free in tw_timer_handler():

BUG: KASAN: use-after-free in tw_timer_handler+0x4a/0xa0 at addr ffff88010e0d1e10
Read of size 8 by task swapper/1/0
Call Trace:
__asan_load8+0x54/0x90
tw_timer_handler+0x4a/0xa0
call_timer_fn+0x127/0x480
expire_timers+0x1db/0x2e0
run_timer_softirq+0x12f/0x2a0
__do_softirq+0x105/0x5b4
irq_exit+0xdd/0xf0
smp_apic_timer_interrupt+0x57/0x70
apic_timer_interrupt+0x90/0xa0

Object at ffff88010e0d1bc0, in cache net_namespace size: 6848
Allocated:
save_stack_trace+0x1b/0x20
kasan_kmalloc+0xee/0x180
kasan_slab_alloc+0x12/0x20
kmem_cache_alloc+0x134/0x310
copy_net_ns+0x8d/0x280
create_new_namespaces+0x23f/0x340
unshare_nsproxy_namespaces+0x75/0xf0
SyS_unshare+0x299/0x4f0
entry_SYSCALL_64_fastpath+0x18/0xad
Freed:
save_stack_trace+0x1b/0x20
kasan_slab_free+0xae/0x180
kmem_cache_free+0xb4/0x350
net_drop_ns+0x3f/0x50
cleanup_net+0x3df/0x450
process_one_work+0x419/0xbb0
worker_thread+0x92/0x850
kthread+0x192/0x1e0
ret_from_fork+0x2e/0x40

Add .exit_batch hook to dccp_v4_ops()/dccp_v6_ops() which will purge
timewait sockets on net namespace destruction and prevent above issue.

Fixes: f2bf415cfed7 ("mib: add net to NET_ADD_STATS_BH")
Reported-by: Dmitry Vyukov <[e-mail]>
Signed-off-by: Andrey Ryabinin <[e-mail]>
Acked-by: Arnaldo Carvalho de Melo <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/dccp/ipv4.c

···
 	inet_ctl_sock_destroy(net->dccp.v4_ctl_sk);
 }
 
+static void __net_exit dccp_v4_exit_batch(struct list_head *net_exit_list)
+{
+	inet_twsk_purge(&dccp_hashinfo, AF_INET);
+}
+
 static struct pernet_operations dccp_v4_ops = {
 	.init	= dccp_v4_init_net,
 	.exit	= dccp_v4_exit_net,
+	.exit_batch = dccp_v4_exit_batch,
 };
 
 static int __init dccp_v4_init(void)

net/dccp/ipv6.c

···
 	inet_ctl_sock_destroy(net->dccp.v6_ctl_sk);
 }
 
+static void __net_exit dccp_v6_exit_batch(struct list_head *net_exit_list)
+{
+	inet_twsk_purge(&dccp_hashinfo, AF_INET6);
+}
+
 static struct pernet_operations dccp_v6_ops = {
 	.init   = dccp_v6_init_net,
 	.exit   = dccp_v6_exit_net,
+	.exit_batch = dccp_v6_exit_batch,
 };
 
 static int __init dccp_v6_init(void)