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

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 7 Apr 2017 18:27:37 +0300
Nikolay Aleksandrov <nikolay@cumulusnetworks.com> wrote:

> On 07/04/17 18:22, Stephen Hemminger wrote:
> > On Fri, 7 Apr 2017 17:19:48 +0300
> > Nikolay Aleksandrov <nikolay@cumulusnetworks.com> wrote:
> >   
> >> On 07/04/17 17:10, Stephen Hemminger wrote:  
> >>> On Fri, 7 Apr 2017 15:49:15 +0300
> >>> <idosch@mellanox.com> wrote:
> >>>     
> >>>> From: Ido Schimmel <idosch@mellanox.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@cumulusnetworks.com>
> >>>> Signed-off-by: Ido Schimmel <idosch@mellanox.com>
> >>>> Reported-by: Peter V. Saveliev <peter@svinota.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.
> >>
> >>
> >>  
> > 
> > Why not just add pointer validation in the PVID attribute parsing.
> >   
> 
> We cannot have the changelink() before the register  for many reasons,
> first the vlan config will not be applied so all of the vlan options
> won't get set even though they're passed, then the changelink() can
> cause more trouble via the STP calls (f.e. br_stp_start) which can use
> br->dev->ifindex (= 0 at that point), also can use br->dev->name (still
> haven't passed validation and is uninitialized, i.e.
> dev_get_valid_name() hasn't been called and we can have format
> specifiers in it), multicast code also has some codepaths that will
> cause various timers to get started...
> 
> Moving changelink() after the register is much safer.
> 

Then just fix error handling. Why not call unregister?

diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index a8f6acd23e30..6d92d3198e8b 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -1165,11 +1165,15 @@ static int br_dev_newlink(struct net *src_net, struct net_device *dev,
 		spin_unlock_bh(&br->lock);
 	}
 
-	err = br_changelink(dev, tb, data);
+	err = register_netdevice(dev);
 	if (err)
 		return err;
 
-	return register_netdevice(dev);
+	err = br_changelink(dev, tb, data);
+	if (err)
+		unregister_netdevice(dev);
+
+	return err;
 }
 
 static size_t br_get_size(const struct net_device *brdev)
-- 
2.11.0