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

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Message-ID: <9b6bc5f1-9e10-4bf1-a6b0-bb6178d771b0@intel.com>
Date: Tue, 29 Oct 2024 17:40:00 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Daniel Yang <danielyangkang@...il.com>
CC: Martin KaFai Lau <martin.lau@...ux.dev>, Daniel Borkmann
	<daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, "Alexei
 Starovoitov" <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, "Eduard
 Zingerman" <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
	<yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, Stanislav Fomichev
	<sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "open
 list:BPF [NETWORKING] (tcx & tc BPF, sock_addr)" <bpf@...r.kernel.org>, "open
 list:BPF [NETWORKING] (tcx & tc BPF, sock_addr)" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	<syzbot+346474e3bf0b26bd3090@...kaller.appspotmail.com>
Subject: Re: [PATCH net] Drop packets with invalid headers to prevent KMSAN
 infoleak

From: Daniel Yang <danielyangkang@...il.com>
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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=346474e3bf0b26bd3090
> Signed-off-by: Daniel Yang <danielyangkang@...il.com>
> ---
>  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