netdev - Re: [PATCH bpf-next v3 1/3] net: Enlarge offset check value from 0xffff to INT_MAX in bpf_skb_load

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Date:   Fri, 15 Apr 2022 21:31:50 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Liu Jian <liujian56@...wei.com>, ast@...nel.org, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, davem@...emloft.net,
        kuba@...nel.org, sdf@...gle.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH bpf-next v3 1/3] net: Enlarge offset check value from
 0xffff to INT_MAX in bpf_skb_load_bytes

On 4/14/22 3:59 PM, Liu Jian wrote:
> The data length of skb frags + frag_list may be greater than 0xffff,
> and skb_header_pointer can not handle negative offset and negative len.
> So here INT_MAX is used to check the validity of offset and len.
> Add the same change to the related function skb_store_bytes.
> 
> Fixes: 05c74e5e53f6 ("bpf: add bpf_skb_load_bytes helper")
> Signed-off-by: Liu Jian <liujian56@...wei.com>
> Acked-by: Song Liu <songliubraving@...com>
> ---
> v2->v3: change nothing
>   net/core/filter.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 64470a727ef7..1571b6bc51ea 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1687,7 +1687,7 @@ BPF_CALL_5(bpf_skb_store_bytes, struct sk_buff *, skb, u32, offset,
>   
>   	if (unlikely(flags & ~(BPF_F_RECOMPUTE_CSUM | BPF_F_INVALIDATE_HASH)))
>   		return -EINVAL;
> -	if (unlikely(offset > 0xffff))
> +	if (unlikely(offset > INT_MAX || len > INT_MAX))

One more follow-up question, len param is checked by the verifier for the provided buffer.
Why is it needed here? Were you able to create e.g. map values of size larger than INT_MAX?
Please provide details. (Other than that, rest looks good.)

>   		return -EFAULT;
>   	if (unlikely(bpf_try_make_writable(skb, offset + len)))
>   		return -EFAULT;
> @@ -1722,7 +1722,7 @@ BPF_CALL_4(bpf_skb_load_bytes, const struct sk_buff *, skb, u32, offset,
>   {
>   	void *ptr;
>   
> -	if (unlikely(offset > 0xffff))
> +	if (unlikely(offset > INT_MAX || len > INT_MAX))
>   		goto err_clear;
>   
>   	ptr = skb_header_pointer(skb, offset, len, to);
>