Re: [PATCH nf] ipvs: more robust refcounting when sync thread starts

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	Hello,

On Wed, 9 Oct 2019, Davide Caratti wrote:

> > 	Another possibility is the missing checks for 
> > ip_vs_use_count_inc() result to cause underflow on ip_vs_use_count_dec().
> 
> thanks for the advice, the unbalance is probably the cause of the splat
> I'm seeing. On a clean/unpatched kernel, I modified ip_vs_use_count() to
> do this:
> 
> WARN_ON(!try_module_get(THIS_MODULE))

	I think, the problem should be in step 2 below:

- step 1: try_stop_module -> try_release_module_ref -> atomic_sub_return
sets module refcnt to 0. After this point the process to remove module
can not be stopped and try_module_get() will fail in atomic_inc_not_zero.

- step 2: there is very small gap this step to occur: commands are
still registered and they should abort if try_module_get() fails.
This is what we see with WARN and oopses: ip_vs_use_count_inc can not
increment refcnt and we later fail in following ip_vs_use_count_dec.
Commands that add resources still do not need _inc + _dec but
those that increment module refcnt need the _inc check.

- step 3: ip_vs_unregister_nl_ioctl unregisters command handlers.
After this step commands are not running.

- step 4: release all module resources added up to step 2.
On module removal we do not expect to see stop_sync_thread()
stopping any running threads. The module can not be removed while
sync is running.

> using the reproducer below, I still see the splat in module_put(), but
> it's preceded by a failure in ip_vs_use_count_inc(), which accidentally is
> the same one I moved in my patch).
> 
> IPVS: Registered protocols (TCP, UDP, SCTP, AH, ESP)
>  IPVS: Connection hash table configured (size=4096, memory=64Kbytes)
>  IPVS: ipvs loaded.
>  IPVS: sync thread started: state = MASTER, mcast_ifn = veth_syncd, syncid = 0, id = 0
>  IPVS: stopping master sync thread 4005 ...
>  IPVS: sync thread started: state = MASTER, mcast_ifn = veth_syncd, syncid = 0, id = 0
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 4014 at net/netfilter/ipvs/ip_vs_ctl.c:235 ip_vs_use_count_inc+0x19/0x20 [ip_vs]
>  Modules linked in: ip_vs(-) nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 veth ip6table_filter ip6_tables iptable_filter binfmt_misc intel_rapl_msr intel_rapl_common crct10dif_pclmul crc32_pclmul ext4 mbcache jbd2 ghash_clmulni_intel snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_intel_nhlt snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm aesni_intel crypto_simd cryptd glue_helper joydev pcspkr snd_timer virtio_balloon snd soundcore i2c_piix4 nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi virtio_net net_failover virtio_blk failover virtio_console qxl drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ata_piix ttm crc32c_intel serio_raw drm virtio_pci libata virtio_ring virtio floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: nf_defrag_ipv6]
>  CPU: 0 PID: 4014 Comm: ipvsadm Not tainted 5.4.0-rc1.upstream+ #741
>  Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
>  RIP: 0010:ip_vs_use_count_inc+0x19/0x20 [ip_vs]
>  Code: e8 bc 5e dc f4 4c 8b 44 24 08 e9 db fd ff ff 66 90 0f 1f 44 00 00 48 c7 c7 00 02 42 c1 e8 5f 3c 9e f4 84 c0 74 04 0f b6 c0 c3 <0f> 0b 0f b6 c0 c3 90 0f 1f 44 00 00 55 48 89 e5 41 57 49 89 f7 41
>  RSP: 0018:ffff88804ba5f2c8 EFLAGS: 00010246
>  RAX: 0000000000000000 RBX: ffff88804f533ca8 RCX: 0000000000000000
>  RDX: 0000000000000000 RSI: 1ffff1100974be3f RDI: ffffffffc1420200
>  RBP: ffff88804ba5f500 R08: fffffbfff71129d1 R09: fffffbfff71129d1
>  R10: 0000000000000001 R11: fffffbfff71129d0 R12: ffff88804f533300
>  R13: 0000000000000000 R14: 0000000000000001 R15: ffff888079442d00
>  FS:  00007fd42ff9c740(0000) GS:ffff888053800000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000fc91d8 CR3: 000000004fe2c002 CR4: 00000000001606f0
>  Call Trace:
>   start_sync_thread+0x1c17/0x23a0 [ip_vs]
>   ip_vs_genl_new_daemon+0x3b0/0x550 [ip_vs]
>   ip_vs_genl_set_daemon+0x295/0x360 [ip_vs]
>   genl_rcv_msg+0x588/0x11e0
>   netlink_rcv_skb+0x121/0x350
>   genl_rcv+0x24/0x40
>   netlink_unicast+0x416/0x5d0
>   netlink_sendmsg+0x67f/0xb50
>   sock_sendmsg+0xe2/0x110
>   ___sys_sendmsg+0x640/0x940
>   __sys_sendmsg+0xd2/0x170
>   do_syscall_64+0xa5/0x4e0
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>  RIP: 0033:0x7fd42f48f930
>  Code: c3 48 8b 05 42 75 2c 00 f7 db 64 89 18 48 83 cb ff eb dd 0f 1f 80 00 00 00 00 83 3d 6d d6 2c 00 00 75 10 b8 2e 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 ae cc 00 00 48 89 04 24
>  RSP: 002b:00007ffcf69ecaa8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
>  RAX: ffffffffffffffda RBX: 0000000000fc2100 RCX: 00007fd42f48f930
>  RDX: 0000000000000000 RSI: 00007ffcf69ecb50 RDI: 0000000000000003
>  RBP: 0000000000fc2010 R08: 0000000000000001 R09: 0000000000fc2150
>  R10: 00007fd42f7577b8 R11: 0000000000000246 R12: 0000000000fc3160
>  R13: 00007ffcf69ecb50 R14: 0000000000000000 R15: 0000000000000000
>  irq event stamp: 3288
>  hardirqs last  enabled at (3287): [<ffffffffb5bb276f>] __local_bh_enable_ip+0xef/0x1b0
>  hardirqs last disabled at (3288): [<ffffffffb5a0556a>] trace_hardirqs_off_thunk+0x1a/0x20
>  softirqs last  enabled at (3286): [<ffffffffc13f3365>] start_sync_thread+0x1c05/0x23a0 [ip_vs]
>  softirqs last disabled at (3284): [<ffffffffc13f32e1>] start_sync_thread+0x1b81/0x23a0 [ip_vs]
>  ---[ end trace 7b4e216440e510e6 ]---
>  IPVS: stopping master sync thread 4016 ...
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 4013 at kernel/module.c:1146 module_put.part.44+0x15b/0x290
>  Modules linked in: ip_vs(-) nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 veth ip6table_filter ip6_tables iptable_filter binfmt_misc intel_rapl_msr intel_rapl_common crct10dif_pclmul crc32_pclmul ext4 mbcache jbd2 ghash_clmulni_intel snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_intel_nhlt snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm aesni_intel crypto_simd cryptd glue_helper joydev pcspkr snd_timer virtio_balloon snd soundcore i2c_piix4 nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi virtio_net net_failover virtio_blk failover virtio_console qxl drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ata_piix ttm crc32c_intel serio_raw drm virtio_pci libata virtio_ring virtio floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: nf_defrag_ipv6]
>  CPU: 0 PID: 4013 Comm: modprobe Tainted: G        W         5.4.0-rc1.upstream+ #741
>  Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
>  RIP: 0010:module_put.part.44+0x15b/0x290
>  Code: 04 25 28 00 00 00 0f 85 18 01 00 00 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 <0f> 0b e9 78 ff ff ff 65 8b 1d 67 83 26 4a 89 db be 08 00 00 00 48
>  RSP: 0018:ffff888050607c78 EFLAGS: 00010297
>  RAX: 0000000000000003 RBX: ffffffffc1420590 RCX: ffffffffb5db0ef9
>  RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffffffffc1420590
>  RBP: 00000000ffffffff R08: fffffbfff82840b3 R09: fffffbfff82840b3
>  R10: 0000000000000001 R11: fffffbfff82840b2 R12: 1ffff1100a0c0f90
>  R13: ffffffffc1420200 R14: ffff88804f533300 R15: ffff88804f533ca0
>  FS:  00007f8ea9720740(0000) GS:ffff888053800000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 00007f3245abe000 CR3: 000000004c28a006 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]
>   __x64_sys_delete_module+0x2d5/0x400
>   do_syscall_64+0xa5/0x4e0
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>  RIP: 0033:0x7f8ea8bf0db7
>  Code: 73 01 c3 48 8b 0d b9 80 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 89 80 2c 00 f7 d8 64 89 01 48
>  RSP: 002b:00007ffcd38d2fe8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
>  RAX: ffffffffffffffda RBX: 0000000002436240 RCX: 00007f8ea8bf0db7
>  RDX: 0000000000000000 RSI: 0000000000000800 RDI: 00000000024362a8
>  RBP: 0000000000000000 R08: 00007f8ea8eba060 R09: 00007f8ea8c658a0
>  R10: 00007ffcd38d2a60 R11: 0000000000000206 R12: 0000000000000000
>  R13: 0000000000000001 R14: 00000000024362a8 R15: 0000000000000000
>  irq event stamp: 4538
>  hardirqs last  enabled at (4537): [<ffffffffb6193dde>] quarantine_put+0x9e/0x170
>  hardirqs last disabled at (4538): [<ffffffffb5a0556a>] trace_hardirqs_off_thunk+0x1a/0x20
>  softirqs last  enabled at (4522): [<ffffffffb6f8ebe9>] sk_common_release+0x169/0x2d0
>  softirqs last disabled at (4520): [<ffffffffb6f8eb3e>] sk_common_release+0xbe/0x2d0
>  ---[ end trace 7b4e216440e510e7 ]---
>  IPVS: ipvs unloaded.
> 
> so, this is try_module_get() in ip_vs_start_sync_thread() failing to refcount ip_vs.

	Yes, refcnt is not increased.

> When the module is unloaded and stop_sync_thread() is called, the refcounts are
> unbalanced and then we see the splat when the last module_put() is called.

	module_put on refcnt 0

> > 	Are there any steps to reproduce?
> 
> yes, a script that does the following:
> 
>         while true; do
>                 if modprobe -r ip_vs; then
>                         sleep 0.5
>                 fi
>         done >/dev/null 2>&1 &
> 
>         while true; do
>                 ipvsadm --start-daemon master --mcast-interface veth_syncd
>         done >/dev/null 2>&1 &
> 
>         while true; do
>                 ipvsadm --stop-daemon master
>         done >/dev/null 2>&1
> 
> 
> I'm about to send a v2 patch where I test the return value of each call to
> ip_vs_use_count_inc(), and jump to the error path of caller functions
> (returning -EAGAIN) on failure of try_module_get(). For
> ip_vs_start_sync_thread(), the resulting code is very similar to what I
> did in the first post, with the only addition of an additional 'goto
> out_unlock;' in case the attempt to refcount the module failed.

	Yes, every place that increments refcnt would 
need early ip_vs_use_count_inc() call with result check. Then check in 
do_ip_vs_set_ctl() would not be needed, any resources we modify would be
released after the ip_vs_unregister_nl_ioctl call.

	For the other files, check is needed in register_ip_vs_pe() and 
register_ip_vs_scheduler(), register_ip_vs_app() should
_inc+check and _dec on failure.

Regards

--
Julian Anastasov <ja@xxxxxx>



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