netfilter: nft_payload: incorrect arithmetics when fetching VLAN header bits

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2161725
Upstream Status: net commit 696e1a4
CVE: CVE-2023-0179

commit 696e1a4
Author: Pablo Neira Ayuso <pablo@netfilter.org>
Date:   Wed Jan 11 17:07:33 2023 +0100

    netfilter: nft_payload: incorrect arithmetics when fetching VLAN header bits

    If the offset + length goes over the ethernet + vlan header, then the
    length is adjusted to copy the bytes that are within the boundaries of
    the vlan_ethhdr scratchpad area. The remaining bytes beyond ethernet +
    vlan header are copied directly from the skbuff data area.

    Fix incorrect arithmetic operator: subtract, not add, the size of the
    vlan header in case of double-tagged packets to adjust the length
    accordingly to address CVE-2023-0179.

    Reported-by: Davide Ornaghi <d.ornaghi97@gmail.com>
    Fixes: f6ae9f1 ("netfilter: nft_payload: add C-VLAN support")
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Signed-off-by: Florian Westphal <fwestpha@redhat.com>

Author: fw-strlen

net/netfilter/nft_payload.c

@@ -62,7 +62,7 @@ nft_payload_copy_vlan(u32 *d, const struct sk_buff *skb, u8 offset, u8 len)
 			return false;
 
 		if (offset + len > VLAN_ETH_HLEN + vlan_hlen)
-			ethlen -= offset + len - VLAN_ETH_HLEN + vlan_hlen;
+			ethlen -= offset + len - VLAN_ETH_HLEN - vlan_hlen;
 
 		memcpy(dst_u8, vlanh + offset - vlan_hlen, ethlen);