答复: [PATCH bpf-next] bpf: Don't redirect packets with invalid pkt_len

[shaozhengchao]

-----邮件原件-----
发件人: Daniel Borkmann [mailto:daniel@xxxxxxxxxxxxx] 
发送时间: 2022年7月13日 4:12
收件人: sdf@xxxxxxxxxx; shaozhengchao <shaozhengchao@xxxxxxxxxx>
抄送: bpf@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx; hawk@xxxxxxxxxx; ast@xxxxxxxxxx; andrii@xxxxxxxxxx; martin.lau@xxxxxxxxx; song@xxxxxxxxxx; yhs@xxxxxx; john.fastabend@xxxxxxxxx; kpsingh@xxxxxxxxxx; weiyongjun (A) <weiyongjun1@xxxxxxxxxx>; yuehaibing <yuehaibing@xxxxxxxxxx>
主题: Re: [PATCH bpf-next] bpf: Don't redirect packets with invalid pkt_len

On 7/12/22 6:58 PM, sdf@xxxxxxxxxx wrote:
> On 07/12, Zhengchao Shao wrote:
>> Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout 
>> any skbs, that is, the flow->head is null.
>> The root cause, as the [2] says, is because that 
>> bpf_prog_test_run_skb() run a bpf prog which redirects empty skbs.
>> So we should determine whether the length of the packet modified by 
>> bpf prog or others like bpf_prog_test is valid before forwarding it directly.
> 
>> LINK: [1] 
>> https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d
>> 16ec96c5
>> LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
> 
>> Reported-by: syzbot+7a12909485b94426aceb@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>
>> ---
>>   net/core/filter.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
>> diff --git a/net/core/filter.c b/net/core/filter.c index 
>> 4ef77ec5255e..27801b314960 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -2122,6 +2122,11 @@ static int __bpf_redirect_no_mac(struct 
>> sk_buff *skb, struct net_device *dev,
>>   {
>>       unsigned int mlen = skb_network_offset(skb);
> 
>> +    if (unlikely(skb->len == 0)) {
>> +        kfree_skb(skb);
>> +        return -EINVAL;
>> +    }
>> +
>>       if (mlen) {
>>           __skb_pull(skb, mlen);
> 
>> @@ -2143,7 +2148,9 @@ static int __bpf_redirect_common(struct sk_buff 
>> *skb, struct net_device *dev,
>>                    u32 flags)
>>   {
>>       /* Verify that a link layer header is carried */
>> -    if (unlikely(skb->mac_header >= skb->network_header)) {
>> +    if (unlikely(skb->mac_header >= skb->network_header) ||
>> +        (min_t(u32, skb_mac_header_len(skb), skb->len) <
>> +         (u32)dev->min_header_len)) {
> 
> Why check skb->len != 0 above but skb->len < dev->min_header_len here?
> I guess it doesn't make sense in __bpf_redirect_no_mac because we know 
> that mac is empty, but why do we care in __bpf_redirect_common?
> Why not put this check in the common __bpf_redirect?
> 
> Also, it's still not clear to me whether we should bake it into the 
> core stack vs having some special checks from test_prog_run only. I'm 
> assuming the issue is that we can construct illegal skbs with that 
> test_prog_run interface, so maybe start by fixing that?

Agree, ideally we can prevent it right at the source rather than adding more tests into the fast-path.

> Did you have a chance to look at the reproducer more closely? What 
> exactly is it doing?
> 
>>           kfree_skb(skb);
>>           return -ERANGE;
>>       }
>> --
>> 2.17.1

> 


Hi Daniel and sdf:
	Thank you for your reply. I read the poc code carefully, and I think the current call stack is like:
sys_bpf(BPF_PROG_TEST_RUN, &attr, sizeof(attr)) -> bpf_prog_test_run->bpf_prog_test_run_skb.

In function bpf_prog_test_run_skb, procedure will use build_skb to generate a new skb. Poc code pass
a 14Byte packet for direct. First ,skb->len = 14, but after trans eth type, the len = 0; but is_l2 is false, 
so len=0 when run bpf_test_run. Is it possible to add check in convert___skb_to_skb? When skb->len=0,
we drop the packet.

But, if some other paths call bpf redirect with skb->len=0, this is not effective, such as some driver call redirect fuction.
I don't know if I'm thinking right.

Thank you.

Zhengchao Shao