Re: [PATCH net] Drop packets with invalid headers to prevent KMSAN infoleak

[Alexander Lobakin]

From: Daniel Yang <danielyangkang@xxxxxxxxx>
Date: Sat, 19 Oct 2024 00:11:39 -0700

> KMSAN detects uninitialized memory stored to memory by
> bpf_clone_redirect(). Adding a check to the transmission path to find
> malformed headers prevents this issue. Specifically, we check if the length
> of the data stored in skb is less than the minimum device header length.
> If so, drop the packet since the skb cannot contain a valid device header.
> Also check if mac_header_len(skb) is outside the range provided of valid
> device header lengths.
> 
> Testing this patch with syzbot removes the bug.
> 
> Fixes: 88264981f208 ("Merge tag 'sched_ext-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext")
> Reported-by: syzbot+346474e3bf0b26bd3090@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=346474e3bf0b26bd3090
> Signed-off-by: Daniel Yang <danielyangkang@xxxxxxxxx>
> ---
>  net/core/filter.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cd3524cb3..92d8f2098 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2191,6 +2191,13 @@ static int __bpf_redirect_common(struct sk_buff *skb, struct net_device *dev,
>  		return -ERANGE;
>  	}
>  
> +	if (unlikely(skb->len < dev->min_header_len ||
> +		     skb_mac_header_len(skb) < dev->min_header_len ||
> +		     skb_mac_header_len(skb) > dev->hard_header_len)) {
> +		kfree_skb(skb);
> +		return -ERANGE;
> +	}

I believe this should go under IS_ENABLED(CONFIG_KMSAN) or
CONFIG_DEBUG_NET or so to not affect the regular configurations.
Or does this fix some real bug?

> +
>  	bpf_push_mac_rcsum(skb);
>  	return flags & BPF_F_INGRESS ?
>  	       __bpf_rx_skb(dev, skb) : __bpf_tx_skb(dev, skb);

Thanks,
Olek