ipvs: more robust refcounting when sync thread starts

if the IPVS module is removed while the sync daemon is starting, there is
a small possibility for stop_sync_thread() to end successfully just after
start_sync_thread() released sync_mutex, but before increasing the module
refcount. When this race happens, the following warning is seen in dmesg:

 IPVS: stopping master sync thread 14134 ...
 ------------[ cut here ]------------
 WARNING: CPU: 0 PID: 14133 at kernel/module.c:1137 module_put.part.44+0x167/0x2a0
 Modules linked in: ip_vs(-) nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 sch_netem veth ip6table_filter ip6_tables iptable_filter binfmt_misc intel_rapl_msr intel_rapl_common crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_hda_codec ext4 snd_hda_core snd_hwdep mbcache jbd2 snd_seq snd_seq_device snd_pcm aesni_intel crypto_simd cryptd glue_helper joydev snd_timer virtio_balloon pcspkr snd soundcore i2c_piix4 nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi virtio_net virtio_console virtio_blk net_failover failover qxl drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm crc32c_intel serio_raw ata_piix virtio_pci drm virtio_ring virtio libata floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: nf_defrag_ipv6]
 CPU: 0 PID: 14133 Comm: modprobe Tainted: G        W         5.3.0-rc7.upstream+ torvalds#723
 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
 RIP: 0010:module_put.part.44+0x167/0x2a0
 Code: 48 83 c4 68 5b 5d 41 5c 41 5d 41 5e 41 5f c3 89 44 24 28 83 e8 01 89 c5 0f 89 57 ff ff ff 48 c7 c7 60 9c ed 86 e8 32 4c f5 ff <0f> 0b e9 6c ff ff ff 65 8b 1d 6b b2 e6 7a 89 db be 08 00 00 00 48
 RSP: 0018:ffff88804f9b7c78 EFLAGS: 00010282
 RAX: 0000000000000024 RBX: ffffffffc1609690 RCX: ffffffff85a82042
 RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff888053627bec
 RBP: 00000000ffffffff R08: ffffed100a6c6181 R09: ffffed100a6c6181
 R10: 0000000000000001 R11: ffffed100a6c6180 R12: 1ffff11009f36f90
 R13: ffffffffc1609300 R14: ffff88804c0d5500 R15: ffff88804c0d5ea0
 FS:  00007fa1c73f8740(0000) GS:ffff888053600000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007f914f4d345f CR3: 000000004855a005 CR4: 00000000001606f0
 Call Trace:
  stop_sync_thread+0x3a3/0x7c0 [ip_vs]
  ip_vs_sync_net_cleanup+0x13/0x50 [ip_vs]
  ops_exit_list.isra.5+0x94/0x140
  unregister_pernet_operations+0x29d/0x460
  unregister_pernet_device+0x26/0x60
  ip_vs_cleanup+0x11/0x38 [ip_vs]
  do_syscall_64+0xa5/0x4f0
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
 RIP: 0033:0x7fa1c68c9027
 Code: 73 01 c3 48 8b 0d 69 6e 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 39 6e 2c 00 f7 d8 64 89 01 48
 RSP: 002b:00007ffe8b021cf8 EFLAGS: 00000202 ORIG_RAX: 00000000000000b0
 RAX: ffffffffffffffda RBX: 0000000001528240 RCX: 00007fa1c68c9027
 RDX: 0000000000000000 RSI: 0000000000000800 RDI: 00000000015282a8
 RBP: 0000000000000000 R08: 00007fa1c6b91060 R09: 00007fa1c693d1a0
 R10: 00007ffe8b021760 R11: 0000000000000202 R12: 0000000000000000
 R13: 0000000000000001 R14: 00000000015282a8 R15: 0000000000000000

Fix this ensuring that start_sync_thread() takes a refcount for the ip_vs
module earlier, and eventually drop it in the error path.

Reported-by: Zhi Li <[email protected]>
Signed-off-by: Davide Caratti <[email protected]>

net/netfilter/ipvs/ip_vs_sync.c

@@ -1758,6 +1758,9 @@ int start_sync_thread(struct netns_ipvs *ipvs, struct ipvs_sync_daemon_cfg *c,
 	int result = -ENOMEM;
 	u16 mtu, min_mtu;
 
+	/* increase the module use count */
+	ip_vs_use_count_inc();
+
 	IP_VS_DBG(7, "%s(): pid %d\n", __func__, task_pid_nr(current));
 	IP_VS_DBG(7, "Each ip_vs_sync_conn entry needs %zd bytes\n",
 		  sizeof(struct ip_vs_sync_conn_v0));
@@ -1892,9 +1895,6 @@ int start_sync_thread(struct netns_ipvs *ipvs, struct ipvs_sync_daemon_cfg *c,
 	mutex_unlock(&ipvs->sync_mutex);
 	rtnl_unlock();
 
-	/* increase the module use count */
-	ip_vs_use_count_inc();
-
 	return 0;
 
 out:
@@ -1924,11 +1924,16 @@ int start_sync_thread(struct netns_ipvs *ipvs, struct ipvs_sync_daemon_cfg *c,
 		}
 		kfree(ti);
 	}
+
+	/* decrease the module use count */
+	ip_vs_use_count_dec();
 	return result;
 
 out_early:
 	mutex_unlock(&ipvs->sync_mutex);
 	rtnl_unlock();
+	/* decrease the module use count */
+	ip_vs_use_count_dec();
 	return result;
 }