dccp: fix tasklet usage

syzbot reported a crash in tasklet_action_common() caused by dccp.

dccp needs to make sure socket wont disappear before tasklet handler
has completed.

This patch takes a reference on the socket when arming the tasklet,
and moves the sock_put() from dccp_write_xmit_timer() to dccp_write_xmitlet()

kernel BUG at kernel/softirq.c:514!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 17 Comm: ksoftirqd/1 Not tainted 4.17.0-rc3+ #30
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:tasklet_action_common.isra.19+0x6db/0x700 kernel/softirq.c:515
RSP: 0018:ffff8801d9b3faf8 EFLAGS: 00010246
dccp_close: ABORT with 65423 bytes unread
RAX: 1ffff1003b367f6b RBX: ffff8801daf1f3f0 RCX: 0000000000000000
RDX: ffff8801cf895498 RSI: 0000000000000004 RDI: 0000000000000000
RBP: ffff8801d9b3fc40 R08: ffffed0039f12a95 R09: ffffed0039f12a94
dccp_close: ABORT with 65423 bytes unread
R10: ffffed0039f12a94 R11: ffff8801cf8954a3 R12: 0000000000000000
R13: ffff8801d9b3fc18 R14: dffffc0000000000 R15: ffff8801cf895490
FS:  0000000000000000(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2bc28000 CR3: 00000001a08a9000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 tasklet_action+0x1d/0x20 kernel/softirq.c:533
 __do_softirq+0x2e0/0xaf5 kernel/softirq.c:285
dccp_close: ABORT with 65423 bytes unread
 run_ksoftirqd+0x86/0x100 kernel/softirq.c:646
 smpboot_thread_fn+0x417/0x870 kernel/smpboot.c:164
 kthread+0x345/0x410 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
Code: 48 8b 85 e8 fe ff ff 48 8b 95 f0 fe ff ff e9 94 fb ff ff 48 89 95 f0 fe ff ff e8 81 53 6e 00 48 8b 95 f0 fe ff ff e9 62 fb ff ff <0f> 0b 48 89 cf 48 89 8d e8 fe ff ff e8 64 53 6e 00 48 8b 8d e8
RIP: tasklet_action_common.isra.19+0x6db/0x700 kernel/softirq.c:515 RSP: ffff8801d9b3faf8

Fixes: dc841e30eaea ("dccp: Extend CCID packet dequeueing interface")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Cc: dccp@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ccids/ccid2.c

@@ -126,6 +126,16 @@ static void ccid2_change_l_seq_window(struct sock *sk, u64 val)
 						  DCCPF_SEQ_WMAX));
 }
 
+static void dccp_tasklet_schedule(struct sock *sk)
+{
+	struct tasklet_struct *t = &dccp_sk(sk)->dccps_xmitlet;
+
+	if (!test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
+		sock_hold(sk);
+		__tasklet_schedule(t);
+	}
+}
+
 static void ccid2_hc_tx_rto_expire(struct timer_list *t)
 {
 	struct ccid2_hc_tx_sock *hc = from_timer(hc, t, tx_rtotimer);
@@ -166,7 +176,7 @@ static void ccid2_hc_tx_rto_expire(struct timer_list *t)
 
 	/* if we were blocked before, we may now send cwnd=1 packet */
 	if (sender_was_blocked)
-		tasklet_schedule(&dccp_sk(sk)->dccps_xmitlet);
+		dccp_tasklet_schedule(sk);
 	/* restart backed-off timer */
 	sk_reset_timer(sk, &hc->tx_rtotimer, jiffies + hc->tx_rto);
 out:
@@ -706,7 +716,7 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 done:
 	/* check if incoming Acks allow pending packets to be sent */
 	if (sender_was_blocked && !ccid2_cwnd_network_limited(hc))
-		tasklet_schedule(&dccp_sk(sk)->dccps_xmitlet);
+		dccp_tasklet_schedule(sk);
 	dccp_ackvec_parsed_cleanup(&hc->tx_av_chunks);
 }
 

net/dccp/timer.c

@@ -232,6 +232,7 @@ static void dccp_write_xmitlet(unsigned long data)
 	else
 		dccp_write_xmit(sk);
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }
 
 static void dccp_write_xmit_timer(struct timer_list *t)
@@ -240,7 +241,6 @@ static void dccp_write_xmit_timer(struct timer_list *t)
 	struct sock *sk = &dp->dccps_inet_connection.icsk_inet.sk;
 
 	dccp_write_xmitlet((unsigned long)sk);
-	sock_put(sk);
 }
 
 void dccp_init_xmit_timers(struct sock *sk)