[(RFC)] NULL pointer dereference on rmmod iptable_mangle.

This crash happened on a ppc64le system running ltp network tests when ltp script ran "rmmod iptable_mangle".

[213425.602369] BUG: Kernel NULL pointer dereference at 0x00000010
[213425.602388] Faulting instruction address: 0xc008000000550bdc
[213425.602399] Oops: Kernel access of bad area, sig: 11 [#1]
[213425.602409] LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
[213425.602418] Modules linked in: nf_log_ipv4 nf_log_common iptable_mangle(-) iptable_nat nf_nat nf_conntrack iptable_filter ip_tables xt_limit xt_multiport xt_LOG xt_tcpudp nf_defrag_ipv6 nf_defrag_ipv4 x_tables sch_netem tcp_bbr rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver rds dummy sctp crypto_user veth uhid kvm_pr kvm vfio_iommu_spapr_tce vfio_spapr_eeh vfio hci_vhci bluetooth ecdh_generic ecc vhost_net tap vhost_vsock vmw_vsock_virtio_transport_common vhost vsock uinput n_gsm pps_ldisc ppp_synctty ppp_async ppp_generic slip slhc serport brd tun fuse vfat fat xfs ext4 crc16 mbcache jbd2 mlx5_ib ib_uverbs ib_core mlx5_core mlxfw tls loop be2net ibmveth(XX) st sr_mod cdrom lp parport_pc parport nvram xfrm_user joydev binfmt_misc rpadlpar_io(XX) rpaphp(XX) xsk_diag tcp_diag udp_diag raw_diag inet_diag unix_diag af_packet_diag netlink_diag nfsv3 nfs_acl nfs lockd grace sunrpc fscache af_packet rfkill vmx_crypto gf128mul ibmvnic uio_pdrv_genirq crct10dif_vpmsum uio rtc_generic btrfs
[213425.602577]  libcrc32c xor raid6_pq dm_service_time sd_mod ibmvfc(XX) scsi_transport_fc crc32c_vpmsum dm_mirror dm_region_hash dm_log sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod [last unloaded: ipt_REJECT]
[213425.602659] Supported: No, Unreleased kernel
[213425.602671] CPU: 0 PID: 10 Comm: ksoftirqd/0 Tainted: G               X   5.3.18-14-default #1 SLE15-SP2 (unreleased)
[213425.602682] NIP:  c008000000550bdc LR: c008000001de00c8 CTR: c008000000550b48
[213425.602692] REGS: c000000002973250 TRAP: 0380   Tainted: G               X    (5.3.18-14-default)
[213425.602701] MSR:  800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 88082822  XER: 00000001
[213425.602726] CFAR: c008000000551050 IRQMASK: 0
                GPR00: c008000001de00c8 c0000000029734e0 c00800000055d800 c00000087b7c3600
                GPR04: c000000002973768 0000000000000000 0000000000000000 c0000007ab050800
                GPR08: 000000000000000e c0000007ab050814 c000000001558380 c008000001de04e0
                GPR12: c008000000550b48 c0000000021e0000 c00000000016b358 0000000000000100
                GPR16: 000000000000008e 00000000000000a0 0000000000000000 0000000000000005
                GPR20: 0000000000000000 c000000001168fa8 0000000000000000 c0000007ac4b46d4
                GPR24: c000000002973768 c008000000555f80 0000000000000001 c0000000011ee000
                GPR28: c0000000011ee000 c00000087b7c3600 c0000007ab05080e c000000002973768
[213425.602816] NIP [c008000000550bdc] ipt_do_table+0x94/0x980 [ip_tables]
[213425.602827] LR [c008000001de00c8] iptable_mangle_hook+0x50/0x180 [iptable_mangle]
[213425.602835] Call Trace:
[213425.602843] [c0000000029734e0] [c000000002973570] 0xc000000002973570 (unreliable)
[213425.602856] [c000000002973690] [c008000001de00c8] iptable_mangle_hook+0x50/0x180 [iptable_mangle]
[213425.602871] [c0000000029736f0] [c000000000a82b60] nf_hook_slow+0x70/0x140
[213425.602882] [c000000002973740] [c000000000a90cdc] ip_rcv+0xac/0x120
[213425.602894] [c0000000029737c0] [c0000000009d978c] __netif_receive_skb_core+0x42c/0x1160
[213425.602906] [c0000000029738a0] [c0000000009dab80] __netif_receive_skb_list_core+0x130/0x330
[213425.602919] [c000000002973940] [c0000000009dafa4] netif_receive_skb_list_internal+0x224/0x350
[213425.602932] [c0000000029739c0] [c0000000009db2b4] gro_normal_list.part.109+0x34/0x60
[213425.602943] [c0000000029739f0] [c0000000009dc0c8] napi_gro_receive+0x1b8/0x200
[213425.602957] [c000000002973a30] [c008000000e32368] ibmvnic_poll+0x2d0/0x410 [ibmvnic]
[213425.602969] [c000000002973b10] [c0000000009dcebc] net_rx_action+0x1ec/0x540
[213425.602982] [c000000002973c30] [c000000000c1ff68] __do_softirq+0x178/0x424
[213425.602994] [c000000002973d20] [c00000000013c924] run_ksoftirqd+0x64/0x90
[213425.603006] [c000000002973d40] [c0000000001717c0] smpboot_thread_fn+0x270/0x2c0
[213425.603018] [c000000002973db0] [c00000000016b4fc] kthread+0x1ac/0x1c0
[213425.603029] [c000000002973e20] [c00000000000b660] ret_from_kernel_thread+0x5c/0x7c
[213425.603038] Instruction dump:
[213425.603046] e8e300c0 82c40000 e92d1178 f9210118 39200000 2fbc0000 7fc74214 419e046c
[213425.603067] eb380010 2fb90000 419e0474 393e0006 <80850010> 38c00000 7d404e2c 39200001
[213425.603089] ---[ end trace f2babb2170f723cc ]---
[213425.690517]

In the crash we find in iptable_mangle_hook() that state->net->ipv4.iptable_mangle=NULL causing a NULL pointer dereference. net->ipv4.iptable_mangle is set to NULL in iptable_mangle_net_exit() and called when ip_mangle modules is unloaded. A rmmod task was found in the crash dump.  A 2nd crash showed the same problem when running "rmmod iptable_filter" (net->ipv4.iptable_filter=NULL).

Once a hook is registered packets will picked up a pointer from: net->ipv4.iptable_$table. The patch adds a call to synchronize_net() in ipt_unregister_table() to insure no packets are in flight that have picked up the pointer before completing the un-register.

This change has has prevented the problem in our testing.  However, we have concerns with this change as it would mean that on netns cleanup, we would need one synchronize_net() call for every table in use. Also, on module unload, there would be one synchronize_net() for every existing netns.

Signed-off-by: David Wilder <dwilder@us.ibm.com>
---
 net/ipv4/netfilter/ip_tables.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)