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

[Eric Dumazet]

On Mon, Nov 4, 2024 at 5:02 AM Daniel Yang <danielyangkang@xxxxxxxxx> wrote:
>
> 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.
>
> Macro added to not affect normal builds.
>
> 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>
> ---
> v1: Enclosed in macro to not affect normal builds
>
>  net/core/filter.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cd3524cb3..9c5786f9c 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2191,6 +2191,14 @@ static int __bpf_redirect_common(struct sk_buff *skb, struct net_device *dev,
>                 return -ERANGE;
>         }
>
> +#if IS_ENABLED(CONFIG_KMSAN)
> +       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;
> +       }
> +#endif
>         bpf_push_mac_rcsum(skb);
>         return flags & BPF_F_INGRESS ?
>                __bpf_rx_skb(dev, skb) : __bpf_tx_skb(dev, skb);
> --
> 2.39.2
>

I am not a BPF maintainer, but for the record I think it is wrong to
silence KMSAN and give the impression a bug is 'removed'.