netdev - Re: [PATCH net] bridge: netlink: register netdevice before executing changelink

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Message-ID: <3d567660-7fa5-979e-3097-89427270e554@cumulusnetworks.com>
Date:   Fri, 7 Apr 2017 17:19:48 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Stephen Hemminger <stephen@...workplumber.org>, idosch@...lanox.com
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        bridge@...ts.linux-foundation.org, mlxsw@...lanox.com,
        peter@...nota.eu, cera@...a.cz
Subject: Re: [PATCH net] bridge: netlink: register netdevice before executing
 changelink

On 07/04/17 17:10, Stephen Hemminger wrote:
> On Fri, 7 Apr 2017 15:49:15 +0300
> <idosch@...lanox.com> wrote:
> 
>> From: Ido Schimmel <idosch@...lanox.com>
>>
>> Peter reported a kernel oops when executing the following command:
>>
>> $ ip link add name test type bridge vlan_default_pvid 1
>>
>> [13634.939408] BUG: unable to handle kernel NULL pointer dereference at
>> 0000000000000190
>> [13634.939436] IP: __vlan_add+0x73/0x5f0
>> [...]
>> [13634.939783] Call Trace:
>> [13634.939791]  ? pcpu_next_unpop+0x3b/0x50
>> [13634.939801]  ? pcpu_alloc+0x3d2/0x680
>> [13634.939810]  ? br_vlan_add+0x135/0x1b0
>> [13634.939820]  ? __br_vlan_set_default_pvid.part.28+0x204/0x2b0
>> [13634.939834]  ? br_changelink+0x120/0x4e0
>> [13634.939844]  ? br_dev_newlink+0x50/0x70
>> [13634.939854]  ? rtnl_newlink+0x5f5/0x8a0
>> [13634.939864]  ? rtnl_newlink+0x176/0x8a0
>> [13634.939874]  ? mem_cgroup_commit_charge+0x7c/0x4e0
>> [13634.939886]  ? rtnetlink_rcv_msg+0xe1/0x220
>> [13634.939896]  ? lookup_fast+0x52/0x370
>> [13634.939905]  ? rtnl_newlink+0x8a0/0x8a0
>> [13634.939915]  ? netlink_rcv_skb+0xa1/0xc0
>> [13634.939925]  ? rtnetlink_rcv+0x24/0x30
>> [13634.939934]  ? netlink_unicast+0x177/0x220
>> [13634.939944]  ? netlink_sendmsg+0x2fe/0x3b0
>> [13634.939954]  ? _copy_from_user+0x39/0x40
>> [13634.939964]  ? sock_sendmsg+0x30/0x40
>> [13634.940159]  ? ___sys_sendmsg+0x29d/0x2b0
>> [13634.940326]  ? __alloc_pages_nodemask+0xdf/0x230
>> [13634.940478]  ? mem_cgroup_commit_charge+0x7c/0x4e0
>> [13634.940592]  ? mem_cgroup_try_charge+0x76/0x1a0
>> [13634.940701]  ? __handle_mm_fault+0xdb9/0x10b0
>> [13634.940809]  ? __sys_sendmsg+0x51/0x90
>> [13634.940917]  ? entry_SYSCALL_64_fastpath+0x1e/0xad
>>
>> The problem is that the bridge's VLAN group is created after setting the
>> default PVID, when registering the netdevice and executing its
>> ndo_init().
>>
>> Fix this by changing the order of both operations, so that
>> br_changelink() is only processed after the netdevice is registered,
>> when the VLAN group is already initialized.
>>
>> The changelink() call is done on a best-effort basis since unregistering
>> the netdevice upon failure won't perform a proper cleanup due to a
>> missing ndo_uninit(), which I'll try to add for net-next.
>>
>> Fixes: b6677449dff6 ("bridge: netlink: call br_changelink() during br_dev_newlink()")
>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> Signed-off-by: Ido Schimmel <idosch@...lanox.com>
>> Reported-by: Peter V. Saveliev <peter@...nota.eu>
>> ---
>> Please consider this for 4.4.y, 4.9.y and 4.10.y as well.
> 
> Although this patch fixes the OOPS it breaks all the error handling
> of br_changelink. If bad attributes are passed to newlink, you leave a
> broken partially configured bridge device. The code needs to cleanup
> and return the correct errno.
> 

The cleanup would require adding ndo_uninit() and a much bigger churn
which doesn't seem okay for -net, it will be targetted at net-next.
The bridge can always be reconfigured as all of the options can be set
during runtime, so anything can be fixed, thus the best-effort changelink.

If it is not desirable for -net then maybe we should just revert the
patch there altogether and make it again correctly with cleanup and so
on in net-next.