netdev - Re: [PATCH net] net: rtnetlink: fix bugs in rtnl_alt

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Date:   Thu, 13 Feb 2020 23:11:26 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>, Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Jiri Pirko <jiri@...lanox.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net: rtnetlink: fix bugs in rtnl_alt_ifname()



On 2/12/20 10:58 PM, Eric Dumazet wrote:
> 
> 
> On 2/12/20 10:45 PM, Jiri Pirko wrote:
>> Thu, Feb 13, 2020 at 05:58:26AM CET, edumazet@...gle.com wrote:
>>> Since IFLA_ALT_IFNAME is an NLA_STRING, we have no
>>> guarantee it is nul terminated.
>>>
>>> We should use nla_strdup() instead of kstrdup(), since this
>>> helper will make sure not accessing out-of-bounds data.
>>>
>>>
>>> Fixes: 36fbf1e52bd3 ("net: rtnetlink: add linkprop commands to add and delete alternative ifnames")
>>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>>> Cc: Jiri Pirko <jiri@...lanox.com>
>>> Reported-by: syzbot <syzkaller@...glegroups.com>
>>> ---
>>> net/core/rtnetlink.c | 26 ++++++++++++--------------
>>> 1 file changed, 12 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
>>> index 09c44bf2e1d28842d77b4ed442ef2c051a25ad21..e1152f4ffe33efb0a69f17a1f5940baa04942e5b 100644
>>> --- a/net/core/rtnetlink.c
>>> +++ b/net/core/rtnetlink.c
>>> @@ -3504,27 +3504,25 @@ static int rtnl_alt_ifname(int cmd, struct net_device *dev, struct nlattr *attr,
>>> 	if (err)
>>> 		return err;
>>>
>>> -	alt_ifname = nla_data(attr);
>>> +	alt_ifname = nla_strdup(attr, GFP_KERNEL);
>>> +	if (!alt_ifname)
>>> +		return -ENOMEM;
>>> +
>>> 	if (cmd == RTM_NEWLINKPROP) {
>>> -		alt_ifname = kstrdup(alt_ifname, GFP_KERNEL);
>>> -		if (!alt_ifname)
>>> -			return -ENOMEM;
>>> 		err = netdev_name_node_alt_create(dev, alt_ifname);
>>> -		if (err) {
>>> -			kfree(alt_ifname);
>>> -			return err;
>>> -		}
>>> +		if (!err)
>>> +			alt_ifname = NULL;
>>> 	} else if (cmd == RTM_DELLINKPROP) {
>>> 		err = netdev_name_node_alt_destroy(dev, alt_ifname);
>>> -		if (err)
>>> -			return err;
>>> 	} else {
>>
>>
>>> -		WARN_ON(1);
>>> -		return 0;
>>> +		WARN_ON_ONCE(1);
>>> +		err = -EINVAL;
>>
>> These 4 lines do not seem to be related to the rest of the patch. Should
>> it be a separate patch?
> 
> Well, we have to kfree(alt_ifname).
> 
> Generally speaking I tried to avoid return in the middle of this function.
> 
> The WARN_ON(1) is dead code today, making it a WARN_ON_ONCE(1) is simply
> a matter of avoiding syslog floods if this path is ever triggered in the future.

Also, related to this new fancy code ;)

Is there anything preventing netdev_name_node_alt_destroy() from destroying the primary
ifname ?

netdev_name_node_lookup() should be able to find dev->name_node itself ?

Then we would leave a dangling pointer in dev->name_node, and crash later.

I am thinking we need this fix :

diff --git a/net/core/dev.c b/net/core/dev.c
index a6316b336128cdb31eea6e80f1a47620abbd0d31..3fa2bc2c30ee1350b5b4b400f0552b9bf2a62697 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -331,6 +331,11 @@ int netdev_name_node_alt_destroy(struct net_device *dev, const char *name)
        name_node = netdev_name_node_lookup(net, name);
        if (!name_node)
                return -ENOENT;
+
+       /* Do not trust users, ever ! */
+       if (name_node == dev->name_node || name_node->dev != dev)
+               return -EINVAL;
+
        __netdev_name_node_alt_destroy(name_node);
 
        return 0;