Re: WARNING: CPU: 2 PID: 4277 at lib/refcount.c:186

[Eric Dumazet]

On Fri, 2017-09-08 at 10:21 -0700, Cong Wang wrote:
> (Cc'ing netdev)
> 
> On Fri, Sep 8, 2017 at 5:59 AM, Shankara Pailoor <sp3485@xxxxxxxxxxxx> wrote:
> > Hi,
> >
> > I found a warning while fuzzing with Syzkaller on linux 4.13-rc7 on
> > x86_64. The full stack trace is below:
> >
> > 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
> >
> >
> >
> >
> > I found that the following program is able to reproduce the warning:
> >
> >
> > Pastebin: https://pastebin.com/B75BdYKz
> >
> > Here are my configs: https://pastebin.com/zRYCXbak
> >
> > Regards,
> > Shankara
> >

Sweet, thanks for the report, I will have a look.

It seems one reqsk_put(req); is missing, but that would lead to a memory
leak, not a double reqsk_put(req) :/

( Same fix is needed for IPv6 )

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a63486afa7a7e7b4dce88b65bc27cfa872a3ba2f..d9416b5162bc1bdd1acd34fcb4da21cb6b62d0ae 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1669,9 +1669,9 @@ int tcp_v4_rcv(struct sk_buff *skb)
                 */
                sock_hold(sk);
                refcounted = true;
-               if (tcp_filter(sk, skb))
-                       goto discard_and_relse;
-               nsk = tcp_check_req(sk, skb, req, false);
+               nsk = NULL;
+               if (!tcp_filter(sk, skb))
+                       nsk = tcp_check_req(sk, skb, req, false);
                if (!nsk) {
                        reqsk_put(req);
                        goto discard_and_relse;