Re: [PATCH net] net: tun: fix bugs for oversize packet when napi frags enabled

[Ziyang Xuan (William)]

> On Fri, Oct 28, 2022 at 8:32 PM Ziyang Xuan
> <william.xuanziyang@xxxxxxxxxx> wrote:
>>
>> Recently, we got two syzkaller problems because of oversize packet
>> when napi frags enabled.
>>
>> One of the problems is because the first seg size of the iov_iter
>> from user space is very big, it is 2147479538 which is bigger than
>> the threshold value for bail out early in __alloc_pages(). And
>> skb->pfmemalloc is true, __kmalloc_reserve() would use pfmemalloc
>> reserves without __GFP_NOWARN flag. Thus we got a warning as following:
>>
>> ========================================================
>>
> 
>> Restrict the packet size less than ETH_MAX_MTU to fix the problems.
>> Add len check in tun_napi_alloc_frags() simply. Athough that makes
>> some kinds of packets payload size slightly smaller than the length
>> allowed by the protocol, for example, ETH_HLEN + sizeof(struct ipv6hdr)
>> smaller when the tun device type is IFF_TAP and the packet is IPv6. But
>> I think that the effect is small and can be ignored.
> 
> I am not sure about ETH_MAX_MTU being completely safe.
> 
> napi_get_frags() / napi_alloc_skb() is reserving NET_SKB_PAD +
> NET_IP_ALIGN bytes.
> 
> transport_header being an offset from skb->head,
> we probably want to use (ETH_MAX_MTU - NET_SKB_PAD - NET_IP_ALIGN)

Hi Eric,

Thank you for your review. I did not notice the reserved skb space.
I will fix it in v2 patch.

Thanks.
> 
> My objection to your initial patch was that you were using PAGE_SIZE,
> while Ethernet MTU can easily be ~9000
> 
> But 0xFFFF is a bit too much/risky.
> 
> Thanks.
> 
>>
>> Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
>> Signed-off-by: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
>> ---
>>  drivers/net/tun.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 27c6d235cbda..98d3160fcae2 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1459,7 +1459,7 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
>>         int err;
>>         int i;
>>
>> -       if (it->nr_segs > MAX_SKB_FRAGS + 1)
>> +       if (it->nr_segs > MAX_SKB_FRAGS + 1 || len > ETH_MAX_MTU)
>>                 return ERR_PTR(-EMSGSIZE);
>>
>>         local_bh_disable();
>> --
>> 2.25.1
>>
> .
>