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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Simon Horman , sd@queasysnail.net, Vladimir Oltean , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH net v2] net/packet: reset the MAC header on the packet-socket transmit path Date: Fri, 24 Jul 2026 16:40:15 +0200 Message-ID: <20260724144015.63219-1-doruk@0sec.ai> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" packet_parse_headers() resets the MAC header only for a SOCK_RAW frame whose socket did not bind a protocol. A protocol-bound SOCK_RAW socket, any SOCK_DGRAM frame, and the legacy SOCK_PACKET path therefore leave skb->mac_header unset here. For frames sent via __dev_queue_xmit() this is harmless: it resets the MAC header unconditionally. But the packet-socket PACKET_QDISC_BYPASS path uses dev_direct_xmit(), which does not, so the frame reaches ndo_start_xmit() with the MAC header unset. A driver that reads eth_hdr(skb) on transmit then dereferences skb->head + (u16)~0, an out-of-bounds access ~64 KiB past the head -- the same class fixed for one consumer in commit f5089008f90c ("macsec: do not read an unset MAC header in macsec_encrypt()"). packet_parse_headers() runs only on the transmit path, where skb->data points at the start of the L2 header for every packet-socket type regardless of its length: SOCK_RAW and SOCK_PACKET carry a user-supplied header and SOCK_DGRAM has one built by dev_hard_header(). Reset the MAC header unconditionally, mirroring __dev_queue_xmit(), so the frame is anchored on the bypass path too. Found by 0sec (https://0sec.ai) using automated source analysis; verified against source and matched to the macsec KASAN report in f5089008f90c. Compile-tested. Fixes: 75c65772c3d1 ("net/packet: Ask driver for protocol if not provided b= y user") Cc: stable@vger.kernel.org Assisted-by: 0sec:multi-model Signed-off-by: Doruk Tan Ozturk --- v2: address Willem de Bruijn's review -- clarify that __dev_queue_xmit() already resets so only the dev_direct_xmit()/PACKET_QDISC_BYPASS path is affected; note skb->data is the L2 start for all packet-socket TX types regardless of L2 length; move the explanatory comment out of the code into the commit log (kept one terse line). net/packet/af_packet.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index e75d2932475a..5ae0511e89e3 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -1924,11 +1924,12 @@ static void packet_parse_headers(struct sk_buff *sk= b, struct socket *sock) { int depth; =20 + /* On TX skb->data is the L2 header; anchor it for all socket types. */ + skb_reset_mac_header(skb); + if ((!skb->protocol || skb->protocol =3D=3D htons(ETH_P_ALL)) && - sock->type =3D=3D SOCK_RAW) { - skb_reset_mac_header(skb); + sock->type =3D=3D SOCK_RAW) skb->protocol =3D dev_parse_header_protocol(skb); - } =20 /* Move network header to the right position for VLAN tagged packets */ if (likely(skb->dev->type =3D=3D ARPHRD_ETHER) && --=20 2.43.0