device lo entered promiscuous mode
TCP: request_sock_TCPv6: Possible SYN flooding on port 20002. Sending cookies.  Check SNMP counters.

======================================================
WARNING: possible circular locking dependency detected
4.17.0-rc1+ #13 Not tainted
------------------------------------------------------
syzkaller848376/4484 is trying to acquire lock:
00000000eb09d70b (sk_lock-AF_INET6){+.+.}, at: lock_sock include/net/sock.h:1469 [inline]
00000000eb09d70b (sk_lock-AF_INET6){+.+.}, at: tcp_mmap+0x1c7/0x14f0 net/ipv4/tcp.c:1759

but task is already holding lock:
000000006af736a5 (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0x1a1/0x2a0 mm/util.c:355

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&mm->mmap_sem){++++}:
       __might_fault+0x155/0x1e0 mm/memory.c:4555
       _copy_from_iter_full+0x2fd/0xd10 lib/iov_iter.c:607
       copy_from_iter_full include/linux/uio.h:124 [inline]
       skb_do_copy_data_nocache include/net/sock.h:1883 [inline]
       skb_add_data_nocache include/net/sock.h:1894 [inline]
       tcp_sendmsg_locked+0x2f98/0x3e10 net/ipv4/tcp.c:1316
       tcp_sendmsg+0x2f/0x50 net/ipv4/tcp.c:1446
       inet_sendmsg+0x19f/0x690 net/ipv4/af_inet.c:798
       sock_sendmsg_nosec net/socket.c:629 [inline]
       sock_sendmsg+0xd5/0x120 net/socket.c:639
       __sys_sendto+0x3d7/0x670 net/socket.c:1789
       __do_sys_sendto net/socket.c:1801 [inline]
       __se_sys_sendto net/socket.c:1797 [inline]
       __x64_sys_sendto+0xe1/0x1a0 net/socket.c:1797
       do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (sk_lock-AF_INET6){+.+.}:
       lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
       lock_sock_nested+0xd0/0x120 net/core/sock.c:2844
       lock_sock include/net/sock.h:1469 [inline]
       tcp_mmap+0x1c7/0x14f0 net/ipv4/tcp.c:1759
       sock_mmap+0x8e/0xc0 net/socket.c:1144
       call_mmap include/linux/fs.h:1789 [inline]
       mmap_region+0xd13/0x1820 mm/mmap.c:1723
       do_mmap+0xc79/0x11d0 mm/mmap.c:1494
       do_mmap_pgoff include/linux/mm.h:2237 [inline]
       vm_mmap_pgoff+0x1fb/0x2a0 mm/util.c:357
       ksys_mmap_pgoff+0x4c9/0x640 mm/mmap.c:1544
       __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
       __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
       __x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
       do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&mm->mmap_sem);
                               lock(sk_lock-AF_INET6);
                               lock(&mm->mmap_sem);
  lock(sk_lock-AF_INET6);

 *** DEADLOCK ***

1 lock held by syzkaller848376/4484:
 #0: 000000006af736a5 (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0x1a1/0x2a0 mm/util.c:355

stack backtrace:
CPU: 0 PID: 4484 Comm: syzkaller848376 Not tainted 4.17.0-rc1+ #13
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+0x1b9/0x294 lib/dump_stack.c:113
 print_circular_bug.isra.36.cold.54+0x1bd/0x27d kernel/locking/lockdep.c:1223
 check_prev_add kernel/locking/lockdep.c:1863 [inline]
 check_prevs_add kernel/locking/lockdep.c:1976 [inline]
 validate_chain kernel/locking/lockdep.c:2417 [inline]
 __lock_acquire+0x343e/0x5140 kernel/locking/lockdep.c:3431
 lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
 lock_sock_nested+0xd0/0x120 net/core/sock.c:2844
 lock_sock include/net/sock.h:1469 [inline]
 tcp_mmap+0x1c7/0x14f0 net/ipv4/tcp.c:1759
 sock_mmap+0x8e/0xc0 net/socket.c:1144
 call_mmap include/linux/fs.h:1789 [inline]
 mmap_region+0xd13/0x1820 mm/mmap.c:1723
 do_mmap+0xc79/0x11d0 mm/mmap.c:1494
 do_mmap_pgoff include/linux/mm.h:2237 [inline]
 vm_mmap_pgoff+0x1fb/0x2a0 mm/util.c:357
 ksys_mmap_pgoff+0x4c9/0x640 mm/mmap.c:1544
 __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
 __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
 __x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x43fef9
RSP: 002b:00007ffe6e12e8c8 EFLAGS: 00000203 ORIG_RAX: 0000000000000009
RAX