Re: [nf-next PATCH v2] netfilter: nft_exthdr: Fix for unsafe packet data read

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Jun 08, 2021 at 11:40:57AM +0200, Phil Sutter wrote:
> While iterating through an SCTP packet's chunks, skb_header_pointer() is
> called for the minimum expected chunk header size. If (that part of) the
> skbuff is non-linear, the following memcpy() may read data past
> temporary buffer '_sch'. Use skb_copy_bits() instead which does the
> right thing in this situation.
> 
> Fixes: 133dc203d77df ("netfilter: nft_exthdr: Support SCTP chunks")
> Suggested-by: Florian Westphal <fw@strlen.de>
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
> Changes since v1:
> - skb_copy_bits() call error handling added
> ---
>  net/netfilter/nft_exthdr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
> index 1b0579cb62d08..7f705b5c09de8 100644
> --- a/net/netfilter/nft_exthdr.c
> +++ b/net/netfilter/nft_exthdr.c
> @@ -327,7 +327,9 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
>  				break;
>  
>  			dest[priv->len / NFT_REG32_SIZE] = 0;
> -			memcpy(dest, (char *)sch + priv->offset, priv->len);
> +			if (skb_copy_bits(pkt->skb, offset + priv->offset,
> +					  dest, priv->len) < 0)
> +				break;

Hm, it looks like tcp exthdt matching has the same problem?

>  			return;
>  		}
>  		offset += SCTP_PAD4(ntohs(sch->length));
> -- 
> 2.31.1
>