Re: kernel panic in skb_copy_bits

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-06-27 at 10:58 +0800, Joe Jin wrote:
> Hi,
> 
> When we do fail over test with iscsi + multipath by reset the switches
> on OVM(2.6.39) we hit the panic:
> 
> BUG: unable to handle kernel paging request at ffff88006d9e8d48
> IP: [<ffffffff812605bb>] memcpy+0xb/0x120
> PGD 1798067 PUD 1fd2067 PMD 213f067 PTE 0
> Oops: 0000 [#1] SMP 
> CPU 7 
> Modules linked in: dm_nfs tun nfs fscache auth_rpcgss nfs_acl xen_blkback xen_netback xen_gntdev xen_evtchn lockd sunrpc bridge stp llc bonding be2iscsi iscsi_boot_sysfs ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp bnx2i cnic uio ipv6 cxgb3i libcxgbi cxgb3 mdio dm_round_robin dm_multipath libiscsi_tcp libiscsi scsi_transport_iscsi xenfs xen_privcmd video sbs sbshc acpi_memhotplug acpi_ipmi ipmi_msghandler parport_pc lp parport ixgbe dca sr_mod cdrom bnx2 radeon ttm drm_kms_helper drm snd_seq_dummy i2c_algo_bit i2c_core snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss serio_raw snd_pcm snd_timer snd soundcore snd_page_alloc iTCO_wdt pcspkr iTCO_vendor_support pata_acpi dcdbas i5k_amb ata_generic hwmon floppy ghes i5000_edac edac_core hed dm_snapshot dm_zero dm_mirror dm_region_hash dm_log dm_mod usb_storage lpfc scsi_transport_fc scsi_tgt ata_piix sg shpchp mptsas mptscsih mptbase scsi_transport_sas sd_mod crc_t10dif ext3 j!
>  bd mbcache
> 
> 
> Pid: 0, comm: swapper Tainted: G        W   2.6.39-300.32.1.el5uek #1 Dell Inc. PowerEdge 2950/0DP246
> RIP: e030:[<ffffffff812605bb>]  [<ffffffff812605bb>] memcpy+0xb/0x120
> RSP: e02b:ffff8801003c3d58  EFLAGS: 00010246
> RAX: ffff880076b9e280 RBX: ffff8800714d2c00 RCX: 0000000000000057
> RDX: 0000000000000000 RSI: ffff88006d9e8d48 RDI: ffff880076b9e280
> RBP: ffff8801003c3dc0 R08: 00000000000bf723 R09: 0000000000000000
> R10: 0000000000000000 R11: 000000000000000a R12: 0000000000000034
> R13: 0000000000000034 R14: 00000000000002b8 R15: 00000000000005a8
> FS:  00007fc1e852a6e0(0000) GS:ffff8801003c0000(0000) knlGS:0000000000000000
> CS:  e033 DS: 002b ES: 002b CR0: 000000008005003b
> CR2: ffff88006d9e8d48 CR3: 000000006370b000 CR4: 0000000000002660
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process swapper (pid: 0, threadinfo ffff880077ac0000, task ffff880077abe240)
> Stack:
>  ffffffff8142db21 0000000000000000 ffff880076b9e280 ffff8800637097f0
>  000002ec00000000 00000000000002b8 ffff880077ac0000 0000000000000000
>  ffff8800637097f0 ffff880066c9a7c0 00000000fffffdb4 000000000000024c
> Call Trace:
>  <IRQ> 
>  [<ffffffff8142db21>] ? skb_copy_bits+0x1c1/0x2e0
>  [<ffffffff8142f173>] skb_copy+0xf3/0x120
>  [<ffffffff81447fbc>] neigh_timer_handler+0x1ac/0x350
>  [<ffffffff810573fe>] ? account_idle_ticks+0xe/0x10
>  [<ffffffff81447e10>] ? neigh_alloc+0x180/0x180
>  [<ffffffff8107dbaa>] call_timer_fn+0x4a/0x110
>  [<ffffffff81447e10>] ? neigh_alloc+0x180/0x180
>  [<ffffffff8107f82a>] run_timer_softirq+0x13a/0x220
>  [<ffffffff81075c39>] __do_softirq+0xb9/0x1d0
>  [<ffffffff810d9678>] ? handle_percpu_irq+0x48/0x70
>  [<ffffffff81511d3c>] call_softirq+0x1c/0x30
>  [<ffffffff810172e5>] do_softirq+0x65/0xa0
>  [<ffffffff8107656b>] irq_exit+0xab/0xc0
>  [<ffffffff812f97d5>] xen_evtchn_do_upcall+0x35/0x50
>  [<ffffffff81511d8e>] xen_do_hypervisor_callback+0x1e/0x30
>  <EOI> 
>  [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
>  [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
>  [<ffffffff8100a0b0>] ? xen_safe_halt+0x10/0x20
>  [<ffffffff8101dfeb>] ? default_idle+0x5b/0x170
>  [<ffffffff81014ac6>] ? cpu_idle+0xc6/0xf0
>  [<ffffffff8100a8c9>] ? xen_irq_enable_direct_reloc+0x4/0x4
>  [<ffffffff814f7bbe>] ? cpu_bringup_and_idle+0xe/0x10
> Code: 01 c6 43 4c 04 19 c0 4c 8b 65 f0 4c 8b 6d f8 83 e0 fc 83 c0 08 88 43 4d 48 8b 5d e8 c9 c3 90 90 48 89 f8 89 d1 c1 e9 03 83 e2 07 <f3> 48 a5 89 d1 f3 a4 c3 20 48 83 ea 20 4c 8b 06 4c 8b 4e 08 4c 
> RIP  [<ffffffff812605bb>] memcpy+0xb/0x120
>  RSP <ffff8801003c3d58>
> CR2: ffff88006d9e8d48
> 
> Reviewed vmcore I found the skb->users is 1 at the moment, checked network neighbour 
> history I found skb_get() be replaced by skb_copy by commit 7e36763b2c:
> 
> commit 7e36763b2c204d59de4e88087f84a2c0c8421f25                               
> Author: Frank Blaschka <frank.blaschka@de.ibm.com>                            
> Date:   Mon Mar 3 12:16:04 2008 -0800
> 
>     [NET]: Fix race in generic address resolution.                            
>         
>     neigh_update sends skb from neigh->arp_queue while neigh_timer_handler    
>     has increased skbs refcount and calls solicit with the                    
>     skb. neigh_timer_handler should not increase skbs refcount but make a     
>     copy of the skb and do solicit with the copy.                             
>     
>     Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>                 
>     Signed-off-by: David S. Miller <davem@davemloft.net>                      
> 
> So can you please give some details of the race? per vmcore seems like the skb data
> be freed, I suspected skb_get() lost at somewhere?
> I reverted above commit the panic not occurred during our testing.
> 
> Any input will appreciate!

Well, fact is that your crash is happening in skb_copy().

Frank patch is OK. I suspect using skb_clone() would work too,
so if these skb were fclone ready, chance of an GFP_ATOMIC allocation
error would be smaller.

So something is providing a wrong skb at the very beginning.

You could try to do a early skb_copy to catch the bug and see in the
stack trace what produced this buggy skb.

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 5c56b21..a7a51fd 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1010,6 +1010,7 @@ int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
 				NEIGH_CACHE_STAT_INC(neigh->tbl, unres_discards);
 			}
 			skb_dst_force(skb);
+			kfree_skb(skb_copy(skb, GFP_ATOMIC));
 			__skb_queue_tail(&neigh->arp_queue, skb);
 			neigh->arp_queue_len_bytes += skb->truesize;
 		}