Re: [PATCH 5.4 020/141] ip6_vti: proper dev_{hold|put} in ndo_[un]init methods

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Thu, May 20, 2021 at 06:16:11AM +0000, Rantala, Tommi T. (Nokia - FI/Espoo) wrote:
> On Mon, 2021-05-17 at 16:01 +0200, Greg Kroah-Hartman wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> [ Upstream commit 40cb881b5aaa0b69a7d93dec8440d5c62dae299f ]
> 
> Hi Greg,
> 
> There's fixup to this commit, hit the "unregister_netdevice" problems in 5.4.120
> while running kernel selftests.
> 
> (also check the "Fixes:" tags, I think not all of them were yet included in 5.4.y)
> 
> 
>   commit 0d7a7b2014b1a499a0fe24c9f3063d7856b5aaaf
>   Author: Eric Dumazet <edumazet@google.com>
>   Date:   Wed Mar 31 14:38:11 2021 -0700
> 
>     ipv6: remove extra dev_hold() for fallback tunnels
>     
>     My previous commits added a dev_hold() in tunnels ndo_init(),
>     but forgot to remove it from special functions setting up fallback tunnels.
>     
>     Fallback tunnels do call their respective ndo_init()
>     
>     This leads to various reports like :
>     
>     unregister_netdevice: waiting for ip6gre0 to become free. Usage count = 2
>     
>     Fixes: 48bb5697269a ("ip6_tunnel: sit: proper dev_{hold|put} in ndo_[un]init methods")
>     Fixes: 6289a98f0817 ("sit: proper dev_{hold|put} in ndo_[un]init methods")
>     Fixes: 40cb881b5aaa ("ip6_vti: proper dev_{hold|put} in ndo_[un]init methods")
>     Fixes: 7f700334be9a ("ip6_gre: proper dev_{hold|put} in ndo_[un]init methods")
>     Signed-off-by: Eric Dumazet <edumazet@google.com>
>     Reported-by: syzbot <syzkaller@googlegroups.com>
>     Signed-off-by: David S. Miller <davem@davemloft.net>
> 
> 
> 
> After adopting CONFIG_PCPU_DEV_REFCNT=n option, syzbot was able to trigger
> a warning [1]
> 
> Issue here is that:
> 
> - all dev_put() should be paired with a corresponding prior dev_hold().
> 
> - A driver doing a dev_put() in its ndo_uninit() MUST also
>   do a dev_hold() in its ndo_init(), only when ndo_init()
>   is returning 0.
> 
> Otherwise, register_netdevice() would call ndo_uninit()
> in its error path and release a refcount too soon.
> 
> Therefore, we need to move dev_hold() call from
> vti6_tnl_create2() to vti6_dev_init_gen()
> 
> [1]
> WARNING: CPU: 0 PID: 15951 at lib/refcount.c:31
> refcount_warn_saturate+0xbf/0x1e0 lib/refcount.c:31
> Modules linked in:
> CPU: 0 PID: 15951 Comm: syz-executor.3 Not tainted 5.12.0-rc4-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:refcount_warn_saturate+0xbf/0x1e0 lib/refcount.c:31
> Code: 1d 6a 5a e8 09 31 ff 89 de e8 8d 1a ab fd 84 db 75 e0 e8 d4 13 ab fd
> 48 c7 c7 a0 e1 c1 89 c6 05 4a 5a e8 09 01 e8 2e 36 fb 04 <0f> 0b eb c4 e8 b8
> 13 ab fd 0f b6 1d 39 5a e8 09 31 ff 89 de e8 58
> RSP: 0018:ffffc90001eaef28 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000040000 RSI: ffffffff815c51f5 RDI: fffff520003d5dd7
> RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
> R10: ffffffff815bdf8e R11: 0000000000000000 R12: ffff88801bb1c568
> R13: ffff88801f69e800 R14: 00000000ffffffff R15: ffff888050889d40
> FS:  00007fc79314e700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f1c1ff47108 CR3: 0000000020fd5000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  __refcount_dec include/linux/refcount.h:344 [inline]
>  refcount_dec include/linux/refcount.h:359 [inline]
>  dev_put include/linux/netdevice.h:4135 [inline]
>  vti6_dev_uninit+0x31a/0x360 net/ipv6/ip6_vti.c:297
>  register_netdevice+0xadf/0x1500 net/core/dev.c:10308
>  vti6_tnl_create2+0x1b5/0x400 net/ipv6/ip6_vti.c:190
>  vti6_newlink+0x9d/0xd0 net/ipv6/ip6_vti.c:1020
>  __rtnl_newlink+0x1062/0x1710 net/core/rtnetlink.c:3443
>  rtnl_newlink+0x64/0xa0 net/core/rtnetlink.c:3491
>  rtnetlink_rcv_msg+0x44e/0xad0 net/core/rtnetlink.c:5553
>  netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2502
>  netlink_unicast_kernel net/netlink/af_netlink.c:1312 [inline]
>  netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1338
>  netlink_sendmsg+0x856/0xd90 net/netlink/af_netlink.c:1927
>  sock_sendmsg_nosec net/socket.c:654 [inline]
>  sock_sendmsg+0xcf/0x120 net/socket.c:674
>  ____sys_sendmsg+0x331/0x810 net/socket.c:2350
>  ___sys_sendmsg+0xf3/0x170 net/socket.c:2404
>  __sys_sendmmsg+0x195/0x470 net/socket.c:2490
>  __do_sys_sendmmsg net/socket.c:2519 [inline]
>  __se_sys_sendmmsg net/socket.c:2516 [inline]
>  __x64_sys_sendmmsg+0x99/0x100 net/socket.c:2516
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  net/ipv6/ip6_vti.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
> index cc6180e08a4f..01ddb0f70c57 100644
> --- a/net/ipv6/ip6_vti.c
> +++ b/net/ipv6/ip6_vti.c
> @@ -192,7 +192,6 @@ static int vti6_tnl_create2(struct net_device *dev)
>  
>         strcpy(t->parms.name, dev->name);
>  
> -       dev_hold(dev);
>         vti6_tnl_link(ip6n, t);
>  
>         return 0;
> @@ -921,6 +920,7 @@ static inline int vti6_dev_init_gen(struct net_device
> *dev)
>         dev->tstats = netdev_alloc_pcpu_stats(struct pcpu_sw_netstats);
>         if (!dev->tstats)
>                 return -ENOMEM;
> +       dev_hold(dev);
>         return 0;
>  }
>  

I do not understand, what needs to be done here?

greg k-h