netfilter: nft_payload: incorrect arithmetics when fetching VLAN header bits

commit 696e1a48b1a1b01edad542a1ef293665864a4dd0 upstream.

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: f6ae9f120dad ("netfilter: nft_payload: add C-VLAN support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c
index 4edd899..d7de2ec 100644 (file)
--- a/net/netfilter/nft_payload.c
+++ b/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);