linux-kernel - Re: [PATCH bpf-next] bpf: Don't redirect packets with invalid pkt

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Date:   Tue, 12 Jul 2022 22:12:16 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     sdf@...gle.com, Zhengchao Shao <shaozhengchao@...wei.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        hawk@...nel.org, ast@...nel.org, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        weiyongjun1@...wei.com, yuehaibing@...wei.com
Subject: Re: [PATCH bpf-next] bpf: Don't redirect packets with invalid pkt_len

On 7/12/22 6:58 PM, sdf@...gle.com 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=0b84da80c2917757915afa89f7738a9d16ec96c5
>> LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html
> 
>> Reported-by: syzbot+7a12909485b94426aceb@...kaller.appspotmail.com
>> Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
>> ---
>>   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
>