From e11e35dfd10960ea8ca4258dfa6ed7aeb207179f Mon Sep 17 00:00:00 2001 From: Siho Lee <25esihoya@gmail.com> Date: Fri, 29 May 2026 00:23:35 +0900 Subject: [PATCH v2] netfilter: nft_payload: validate offset for all csum_type paths When csum_type is NFT_PAYLOAD_CSUM_NONE and csum_flags is 0, the bounds check inside the csum condition block is skipped entirely. 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. This is a valid negative offset that refers to the Ethernet header area (used by bridge/vlan rules). However, without any bounds check in the csum=NONE path: - skb_ensure_writable(skb, max(offset + priv->len, 0)): max() converts negative values to 0, making it a no-op. - skb_store_bits(skb, offset, src, priv->len): A negative offset that exceeds skb headroom writes out of bounds. Add proper validation after the csum condition block: - Negative offsets: ensure they fall within skb_headroom(skb) (bridge/vlan rules legitimately access the Ethernet header) - Positive offsets: ensure offset + len does not exceed skb->len Also remove the max() wrapper from skb_ensure_writable() since the new validation guarantees the offset is within range. Fixes: d5953d680f7e ("netfilter: nft_payload: sanitize offset and length before calling skb_checksum()") Cc: stable@vger.kernel.org Signed-off-by: Siho Lee <25esihoya@gmail.com> --- net/netfilter/nft_payload.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c index 01e13e5255a9..2c891c13bbf5 100644 --- a/net/netfilter/nft_payload.c +++ b/net/netfilter/nft_payload.c @@ -892,7 +892,17 @@ static void nft_payload_set_eval(const struct nft_expr *expr, goto err; } - if (skb_ensure_writable(skb, max(offset + priv->len, 0)) || + /* Negative offset (LL_HEADER with bridge/vlan) must be within headroom. + * Positive offset must be within skb length. + */ + if (offset < 0) { + if (-offset > (int)skb_headroom(skb)) + goto err; + } else if (offset + priv->len > skb->len) { + goto err; + } + + if (skb_ensure_writable(skb, offset + priv->len) || skb_store_bits(skb, offset, src, priv->len) < 0) goto err; -- 2.43.0