Re: [PATCH] net: check skb partial checksum offset after trim

[Vasily Averin <vvs@virtuozzo.com>]

Originally it was reported on Ubuntu 4.14 kernel,
then I've reproduced it on upstream 5.10-rc7.
If I'm right the problem is quite old and should 
affect all maintained stable kernels too.

It seems for me the similar problem can happen in __skb_trim_rcsum().
Also I doubt that that skb_checksum_start_offset(skb) checks in 
__skb_postpull_rcsum() and skb_csum_unnecessary() are correct,
becasue they do not guarantee that skb have correct CHECKSUM_PARTIAL.
Could somebody confirm it?

Thank you,
	Vasily Averin

On 12/11/20 6:00 PM, Vasily Averin wrote:
> syzkaller reproduces BUG_ON in skb_checksum_help():
> tun creates skb with big partial checksum area and small ip packet inside,
> then ip_rcv() decreases skb size of below length of checksummed area,
> then checksum_tg() called via netfilter hook detects incorrect skb:
> 
>         offset = skb_checksum_start_offset(skb);
>         BUG_ON(offset >= skb_headlen(skb));
> 
> This patch drops CHEKSUM_PARTIAL mark when skb is trimmed below
> size of checksummed area.
> Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
> Reported-by: syzbot+7010af67ced6105e5ab6@syzkaller.appspotmail.com
> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
> ---
>  include/linux/skbuff.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index a828cf9..0a9545d 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3419,9 +3419,18 @@ static inline void *skb_push_rcsum(struct sk_buff *skb, unsigned int len)
>  
>  static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
>  {
> +	int ret;
> +
>  	if (likely(len >= skb->len))
>  		return 0;
> -	return pskb_trim_rcsum_slow(skb, len);
> +	ret = pskb_trim_rcsum_slow(skb, len);
> +	if (!ret && (skb->ip_summed == CHECKSUM_PARTIAL)) {
> +		int offset = skb_checksum_start_offset(skb) + skb->csum_offset;
> +
> +		if (offset + sizeof(__sum16) > skb_headlen(skb))
> +			skb->ip_summed = CHECKSUM_NONE;
> +	}
> +	return ret;
>  }
>  
>  static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)
>