Re: [PATCH net] skb_checksum_help: fix out-of-bounds access

[Willem de Bruijn]

Junrui Luo wrote:
> The skb_checksum_help() function does not validate negative offset
> values returned by skb_checksum_start_offset(). This can occur when
> __skb_pull() is called on a packet, increasing the headroom while
> leaving csum_start unchanged.

Do you have a specific example where this happens?
 
> A negative offset causes out-of-bounds memory access:
> - skb_checksum() reads before skb->data when computing the checksum
> - skb_checksum_help() writes before skb->data

I don't think this is true out-of-bounds as long as the data access
starts greater than or equal to skb->head, which it will.

There are known cases where such negative skb offsets are
intentional. I don't think this is one of them, but needs a careful
analysis.

The use in skb_checksum does seem to indicate that this is not
intentional indeed. Checksumming a packet where the L4 header would
lie outside the data.

    csum = skb_checksum(skb, offset, skb->len - offset, 0);

> Add validation to detect and reject negative offsets.
> 
> Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
> Reported-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
> Fixes: 663ead3bb8d5 ("[NET]: Use csum_start offset instead of skb_transport_header")
> Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
> ---
>  net/core/dev.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 9094c0fb8c68..30161b9240a2 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3574,6 +3574,11 @@ int skb_checksum_help(struct sk_buff *skb)
>  
>  	offset = skb_checksum_start_offset(skb);
>  	ret = -EINVAL;
> +	if (unlikely(offset < 0)) {
> +		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
> +		WARN_ONCE(true, "offset (%d) < 0\n", offset);
> +		goto out;
> +	}
>  	if (unlikely(offset >= skb_headlen(skb))) {
>  		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
>  		WARN_ONCE(true, "offset (%d) >= skb_headlen() (%u)\n",
> 
> ---
> base-commit: cfd4039213e7b5a828c5b78e1b5235cac91af53d
> change-id: 20251210-fixes-ef9fa1c91916
> 
> Best regards,
> -- 
> Junrui Luo <moonafterrain@xxxxxxxxxxx>
>