Subject: Re: [PATCH net] net: bridge: fix early call to br_stp_change_bridge_id

From: Stephen Hemminger <[email-masked]>
Date: Sat, 16 Dec 2017 10:32:46 -0800

On Sat, 16 Dec 2017 13:31:36 +0200
Nikolay Aleksandrov <[email-masked]> wrote:

> The early call to br_stp_change_bridge_id in bridge's newlink can cause
> a memory leak if an error occurs during the newlink because the fdb
> entries are not cleaned up if a different lladdr was specified, also
> another minor issue is that it generates fdb notifications with
> ifindex = 0. To remove this special case the call is done after netdev
> register and we cleanup any bridge fdb entries on changelink error.
> That also doesn't slow down normal bridge removal, alternative is to call
> it in its ndo_uninit.
> 
> To reproduce the issue:
> $ ip l add br0 address 00:11:22:33:44:55 type bridge group_fwd_mask 1
> RTNETLINK answers: Invalid argument
> 
> $ rmmod bridge
> [ 1822.142525] =============================================================================
> [ 1822.143640] BUG bridge_fdb_cache (Tainted: G           O    ): Objects remaining in bridge_fdb_cache on __kmem_cache_shutdown()
> [ 1822.144821] -----------------------------------------------------------------------------
> 
> [ 1822.145990] Disabling lock debugging due to kernel taint
> [ 1822.146732] INFO: Slab 0x0000000092a844b2 objects=32 used=2 fp=0x00000000fef011b0 flags=0x1ffff8000000100
> [ 1822.147700] CPU: 2 PID: 13584 Comm: rmmod Tainted: G    B      O     4.15.0-rc2+ #87
> [ 1822.148578] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
> [ 1822.150008] Call Trace:
> [ 1822.150510]  dump_stack+0x78/0xa9
> [ 1822.151156]  slab_err+0xb1/0xd3
> [ 1822.151834]  ? __kmalloc+0x1bb/0x1ce
> [ 1822.152546]  __kmem_cache_shutdown+0x151/0x28b
> [ 1822.153395]  shutdown_cache+0x13/0x144
> [ 1822.154126]  kmem_cache_destroy+0x1c0/0x1fb
> [ 1822.154669]  SyS_delete_module+0x194/0x244
> [ 1822.155199]  ? trace_hardirqs_on_thunk+0x1a/0x1c
> [ 1822.155773]  entry_SYSCALL_64_fastpath+0x23/0x9a
> [ 1822.156343] RIP: 0033:0x7f929bd38b17
> [ 1822.156859] RSP: 002b:00007ffd160e9a98 EFLAGS: 00000202 ORIG_RAX: 00000000000000b0
> [ 1822.157728] RAX: ffffffffffffffda RBX: 00005578316ba090 RCX: 00007f929bd38b17
> [ 1822.158422] RDX: 00007f929bd9ec60 RSI: 0000000000000800 RDI: 00005578316ba0f0
> [ 1822.159114] RBP: 0000000000000003 R08: 00007f929bff5f20 R09: 00007ffd160e8a11
> [ 1822.159808] R10: 00007ffd160e9860 R11: 0000000000000202 R12: 00007ffd160e8a80
> [ 1822.160513] R13: 0000000000000000 R14: 0000000000000000 R15: 00005578316ba090
> [ 1822.161278] INFO: Object 0x000000007645de29 @offset=0
> [ 1822.161666] INFO: Object 0x00000000d5df2ab5 @offset=128
> 
> Fixes: a4b816d8ba1c ("bridge: Change local fdb entries whenever mac address of bridge device changes")
> Signed-off-by: Nikolay Aleksandrov <[email-masked]>
> ---
> Consequently this also would fix the null ptr deref due to the rhashtable
> not being initialized in net-next when br_stp_change_bridge_id is called.
> 
> Toshiaki, any reason you called br_stp_change_bridge_id before
> register_netdevice when you introduced it in 30313a3d5794 ?
> 
>  net/bridge/br_netlink.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)

Thanks for working on this.

I agree that fixing this in ndo_uninit would be wrong. There are less bugs
if init and uninit do logically equivalent steps.

A bridge device can be created either with netlink or ioctl.
This change is also makes both ways of adding MAC have
the same semantics;
If bridge is created with ioctl then the bridge_id (and MAC)
will not be changed until later device is added or MAC address
is set by other operation.

Signed-off-by: Stephen Hemminger <[email-masked]>