[PATCH net] wireguard: device: provide sane limits for mtu setting

[Quan Zhou]

Hi Samir,

I didn't realize this. You convinced me on this one. Thanks!

On 2/14/20 15:11, Samir Nassar wrote:
> Hello,
>
> from https://github.com/google/syzkaller/blob/master/docs/syzbot.md
>
> "syzbot system continuously fuzzes main Linux kernel branches and automatically reports found bugs to kernel mailing lists."
>
> As I understand it, fuzzing is applying changes to inputs to see what breaks and, in this case, fix it.
>
> It doesn't make sense to set an MTU to 0 so why allow the program to crash on setting the MTU to zero instead of giving back a useful error or preventing the crash in other ways.
>
> Providing dev->min_mtu and dev->max_mtu bounds is a nice thing to do.
>
> Samir
>
> On February 14, 2020 7:40:23 AM GMT+01:00, Quan Zhou <quan at posteo.net> wrote:
>> I'm just curious, under what circumstances would people set mtu to
>> zero?
>>
>> On 2/14/20 14:38, Eric Dumazet wrote:
>>> If wireguard device mtu is set to zero, a divide by zero
>>> crash happens in calculate_skb_padding().
>>>
>>> This patch provides dev->min_mtu and dev->max_mtu bounds.
>>>
>>> Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
>>> Signed-off-by: Eric Dumazet <edumazet at google.com>
>>> Reported-by: syzbot <syzkaller at googlegroups.com>
>>> Cc: Jason A. Donenfeld <Jason at zx2c4.com>
>>> Cc: wireguard at lists.zx2c4.com
>>> ---
>>>    drivers/net/wireguard/device.c | 5 +++++
>>>    1 file changed, 5 insertions(+)
>>>
>>> diff --git a/drivers/net/wireguard/device.c
>> b/drivers/net/wireguard/device.c
>>> index
>> 43db442b1373073eaf5e805cfe6cfee15875437a..c02b84cca122d92ee8a81c5efdcf67aada2554d6
>> 100644
>>> --- a/drivers/net/wireguard/device.c
>>> +++ b/drivers/net/wireguard/device.c
>>> @@ -271,9 +271,14 @@ static void wg_setup(struct net_device *dev)
>>>    	dev->features |= WG_NETDEV_FEATURES;
>>>    	dev->hw_features |= WG_NETDEV_FEATURES;
>>>    	dev->hw_enc_features |= WG_NETDEV_FEATURES;
>>> +
>>>    	dev->mtu = ETH_DATA_LEN - MESSAGE_MINIMUM_LENGTH -
>>>    		   sizeof(struct udphdr) -
>>>    		   max(sizeof(struct ipv6hdr), sizeof(struct iphdr));
>>> +	dev->min_mtu = MESSAGE_PADDING_MULTIPLE;
>>> +	dev->max_mtu = ETH_MAX_MTU - MESSAGE_MINIMUM_LENGTH -
>>> +		       sizeof(struct udphdr) -
>>> +		       max(sizeof(struct ipv6hdr), sizeof(struct iphdr));
>>>    
>>>    	SET_NETDEV_DEVTYPE(dev, &device_type);
>>>    
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