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

[Rantala, Tommi T. (Nokia - FI/Espoo)]

On Mon, 2021-05-17 at 16:01 +0200, Greg Kroah-Hartman wrote:
From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ 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@xxxxxxxxxx>
  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@xxxxxxxxxx>
    Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>



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@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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;
 }