Back in linux-4.4, I inadvertently put a call to reqsk_put() in
inet_child_forget(), forgetting it could be called from two different
points.

In the case it is called from inet_csk_reqsk_queue_add(), we want to
keep the reference on the request socket, since it is released later by
the caller (tcp_v{4|6}_rcv())

This bug never showed up because atomic_dec_and_test() was not signaling
the underflow, and SLAB_DESTROY_BY RCU semantic for request sockets
prevented the request to be put in quarantine.

Recent conversion of socket refcount from atomic_t to refcount_t finally
exposed the bug.

So move the reqsk_put() to inet_csk_listen_stop() to fix this.

Thanks to Shankara Pailoor for using syzkaller and providing
a nice set of .config and C repro.

WARNING: CPU: 2 PID: 4277 at lib/refcount.c:186
refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:186
Kernel panic - not syncing: panic_on_warn set ...

CPU: 2 PID: 4277 Comm: syz-executor0 Not tainted 4.13.0-rc7 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
Ubuntu-1.8.2-1ubuntu1 04/01/2014
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:16 [inline]
 dump_stack+0xf7/0x1aa lib/dump_stack.c:52
 panic+0x1ae/0x3a7 kernel/panic.c:180
 __warn+0x1c4/0x1d9 kernel/panic.c:541
 report_bug+0x211/0x2d0 lib/bug.c:183
 fixup_bug+0x40/0x90 arch/x86/kernel/traps.c:190
 do_trap_no_signal arch/x86/kernel/traps.c:224 [inline]
 do_trap+0x260/0x390 arch/x86/kernel/traps.c:273
 do_error_trap+0x118/0x340 arch/x86/kernel/traps.c:310
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:323
 invalid_op+0x18/0x20 arch/x86/entry/entry_64.S:846
RIP: 0010:refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:186
RSP: 0018:ffff88006e006b60 EFLAGS: 00010286
RAX: 0000000000000026 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000026 RSI: 1ffff1000dc00d2c RDI: ffffed000dc00d60
RBP: ffff88006e006bf0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 1ffff1000dc00d6d
R13: 00000000ffffffff R14: 0000000000000001 R15: ffff88006ce9d340
 refcount_dec_and_test+0x1a/0x20 lib/refcount.c:211
 reqsk_put+0x71/0x2b0 include/net/request_sock.h:123
 tcp_v4_rcv+0x259e/0x2e20 net/ipv4/tcp_ipv4.c:1729
 ip_local_deliver_finish+0x2e2/0xba0 net/ipv4/ip_input.c:216
 NF_HOOK include/linux/netfilter.h:248 [inline]
 ip_local_deliver+0x1ce/0x6d0 net/ipv4/ip_input.c:257
 dst_input include/net/dst.h:477 [inline]
 ip_rcv_finish+0x8db/0x19c0 net/ipv4/ip_input.c:397
 NF_HOOK include/linux/netfilter.h:248 [inline]
 ip_rcv+0xc3f/0x17d0 net/ipv4/ip_input.c:488
 __netif_receive_skb_core+0x1fb7/0x31f0 net/core/dev.c:4298
 __netif_receive_skb+0x2c/0x1b0 net/core/dev.c:4336
 process_backlog+0x1c5/0x6d0 net/core/dev.c:5102
 napi_poll net/core/dev.c:5499 [inline]
 net_rx_action+0x6d3/0x14a0 net/core/dev.c:5565
 __do_softirq+0x2cb/0xb2d kernel/softirq.c:284
 do_softirq_own_stack+0x1c/0x30 arch/x86/entry/entry_64.S:898
 </IRQ>
 do_softirq.part.16+0x63/0x80 kernel/softirq.c:328
 do_softirq kernel/softirq.c:176 [inline]
 __local_bh_enable_ip+0x84/0x90 kernel/softirq.c:181
 local_bh_enable include/linux/bottom_half.h:31 [inline]
 rcu_read_unlock_bh include/linux/rcupdate.h:705 [inline]
 ip_finish_output2+0x8ad/0x1360 net/ipv4/ip_output.c:231
 ip_finish_output+0x74e/0xb80 net/ipv4/ip_output.c:317
 NF_HOOK_COND include/linux/netfilter.h:237 [inline]
 ip_output+0x1cc/0x850 net/ipv4/ip_output.c:405
 dst_output include/net/dst.h:471 [inline]
 ip_local_out+0x95/0x160 net/ipv4/ip_output.c:124
 ip_queue_xmit+0x8c6/0x1810 net/ipv4/ip_output.c:504
 tcp_transmit_skb+0x1963/0x3320 net/ipv4/tcp_output.c:1123
 tcp_send_ack.part.35+0x38c/0x620 net/ipv4/tcp_output.c:3575
 tcp_send_ack+0x49/0x60 net/ipv4/tcp_output.c:3545
 tcp_rcv_synsent_state_process net/ipv4/tcp_input.c:5795 [inline]
 tcp_rcv_state_process+0x4876/0x4b60 net/ipv4/tcp_input.c:5930
 tcp_v4_do_rcv+0x58a/0x820 net/ipv4/tcp_ipv4.c:1483
 sk_backlog_rcv include/net/sock.h:907 [inline]
 __release_sock+0x124/0x360 net/core/sock.c:2223
 release_sock+0xa4/0x2a0 net/core/sock.c:2715
 inet_wait_for_connect net/ipv4/af_inet.c:557 [inline]
 __inet_stream_connect+0x671/0xf00 net/ipv4/af_inet.c:643
 inet_stream_connect+0x58/0xa0 net/ipv4/af_inet.c:682
 SYSC_connect+0x204/0x470 net/socket.c:1628
 SyS_connect+0x24/0x30 net/socket.c:1609
 entry_SYSCALL_64_fastpath+0x18/0xad
RIP: 0033:0x451e59
RSP: 002b:00007f474843fc08 EFLAGS: 00000216 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000718000 RCX: 0000000000451e59
RDX: 0000000000000010 RSI: 0000000020002000 RDI: 0000000000000007
RBP: 0000000000000046 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000216 R12: 0000000000000000
R13: 00007ffc040a0f8f R14: 00007f47484409c0 R15: 0000000000000000

Fixes: ebb516af ("tcp/dccp: fix race at listener dismantle phase")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Shankara Pailoor <sp3485@columbia.edu>
Tested-by: Shankara Pailoor <sp3485@columbia.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>