Re: BUG in free_netdev() on ppp link deletion

On Tue, Oct 03, 2017 at 09:44:14AM +0200, Beniamino Galvani wrote:

 Call Trace:
  ppp_destroy_interface+0xd8/0xe0 [ppp_generic]
  ppp_disconnect_channel+0xda/0x110 [ppp_generic]
  ppp_unregister_channel+0x5e/0x110 [ppp_generic]
  pppox_unbind_sock+0x23/0x30 [pppox]
  pppoe_connect+0x130/0x440 [pppoe]
  SYSC_connect+0x98/0x110
  ? do_fcntl+0x2c0/0x5d0
  SyS_connect+0xe/0x10
  entry_SYSCALL_64_fastpath+0x1a/0xa5
 RIP: 0033:0x7fa71f4af840
 RSP: 002b:00007ffe4ea40bf8 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
 RAX: ffffffffffffffda RBX: 0000556d37ae0538 RCX: 00007fa71f4af840
 RDX: 000000000000001e RSI: 00007ffe4ea40c00 RDI: 0000000000000008
 RBP: 0000556d37b2a1b0 R08: 0000556d396e95b0 R09: 0000000000000008
 R10: 00000000aaaaaaab R11: 0000000000000246 R12: 0000556d37adc008
 R13: 0000556d37adc004 R14: 0000556d37b2a1a4 R15: 0000000000000000
 Code: 04 00 00 04 e8 cb 52 e3 ff 5b 41 5c 41 5d 5d c3 41 0f b7 84 24 32 02 00 00 4c 89 e7 48 29 c7 e8 80 8b aa ff 5b 41 5c 41 5d 5d c3 <0f> 0b 0f 1f 80 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 
 RIP: free_netdev+0x107/0x110 RSP: ffffc28a40573d88
 ---[ end trace ed294ff0cc40eeff ]---

To reproduce this, establish a PPP connection through pppd, then bring
down and delete the ppp interface:

 # pppd nodetach lock user client plugin rp-pppoe.so ens11 noauth nodeflate password password &
 Plugin rp-pppoe.so loaded.
 RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
 PPP session is 16
 Connected to fe:54:00:5f:04:13 via interface ens11
 Using interface ppp0
 Connect: ppp0 <--> ens11
 CHAP authentication succeeded: Access granted
 CHAP authentication succeeded
 peer from calling number FE:54:00:5F:04:13 authorized
 local  IP address 3.1.1.10
 remote IP address 3.1.1.1

 # ip l set ppp0 down
 # ip l del ppp0

It does not happen every time but only when ppp_destroy_interface() is
called with dev->reg_state = UNREGISTERING, set by the concurrent
rtnl_delete_link().

Indeed, we have a race here: ppp_destroy_interface() can be called before
netdev_run_todo() completes. I'm working on it.