Subject: Re: [PATCH net] netfilter: nft_payload: move offset bounds check outside csum condition

From: Florian Westphal <[email-masked]>
Date: Thu, 28 May 2026 16:12:26 +0200

Siho Lee <[email-masked]> wrote:
> From 574604a1b4a98ee130d7f727ad3c8a7df3f3b6f1 Mon Sep 17 00:00:00 2001
> From: Siho Lee <[email-masked]>
> Date: Thu, 28 May 2026 22:39:03 +0900
> Subject: [PATCH v1] netfilter: nft_payload: move offset bounds check outside
>  csum condition
> 
> The bounds check for offset + priv->len was placed inside the csum
> condition block. When csum_type is NFT_PAYLOAD_CSUM_NONE and
> csum_flags is 0, the entire block including the bounds check is
> skipped.
> 
> For NFT_PAYLOAD_LL_HEADER, offset is computed as:
>     offset = skb_mac_header(skb) - skb->data - vlan_hlen
> which evaluates to -14 (or -18 with VLAN) after eth_type_trans()
> pulls the Ethernet header.
> 
> Without the bounds check, a negative offset reaches:
>     skb_ensure_writable(skb, max(offset + priv->len, 0))
>     skb_store_bits(skb, offset, src, priv->len)
> 
> max(-14 + 4, 0) == 0 makes skb_ensure_writable a no-op, and
> skb_store_bits(skb, -14, ...) writes to skb headroom (OOB write).
> 
> The signed-unsigned comparison in the bounds check correctly catches
> negative offsets: (unsigned int)(-10) is a large positive value that
> exceeds any valid skb->len.
> 
> Move the bounds check outside the csum condition so it applies
> regardless of csum_type/csum_flags.

This breaks link layer update support.

# cd git/nftables/tests/shell
# NFT=nft ./run-tests.sh testcases/packetpath/bridge_pass_up C
W: [FAILED]     1/1 testcases/packetpath/bridge_pass_up