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[34.85.235.172]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-66890523c01sm3800327d50.2.2026.07.23.13.52.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Jul 2026 13:52:10 -0700 (PDT) Date: Thu, 23 Jul 2026 16:52:10 -0400 From: Willem de Bruijn To: Doruk Tan Ozturk , Willem de Bruijn , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Simon Horman , Sabrina Dubroca , Vladimir Oltean , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Doruk Tan Ozturk Message-ID: In-Reply-To: <20260723020337.19040-1-doruk@0sec.ai> References: <20260723020337.19040-1-doruk@0sec.ai> Subject: Re: [PATCH net] net/packet: reset the MAC header on the packet-socket transmit path Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Doruk Tan Ozturk wrote: > packet_parse_headers() resets the MAC header only for a SOCK_RAW frame > whose socket did not bind a protocol: > > if ((!skb->protocol || skb->protocol == htons(ETH_P_ALL)) && > sock->type == SOCK_RAW) { > skb_reset_mac_header(skb); > skb->protocol = dev_parse_header_protocol(skb); > } > > Every other outgoing packet-socket frame therefore reaches > ndo_start_xmit() with the MAC header unset: a SOCK_RAW socket bound to a The normal __dev_queue_xmit calls skb_reset_mac_header. This therefore only applies to the dev_direct_xmit path. > specific protocol (for example socket(AF_PACKET, SOCK_RAW, > htons(ETH_P_IP))), any SOCK_DGRAM frame (its header is built by > dev_hard_header(), which does not set mac_header), and the legacy > SOCK_PACKET path. A driver that reads eth_hdr(skb) on transmit then > dereferences skb->head + (u16)~0, an out-of-bounds access about 64 KiB > past the head. This is not currently an expected invariant, but I see from a comment below the --- that this approach is proposed instead of fixing up the buggy drivers. > > This is the same class fixed for one consumer in commit f5089008f90c > ("macsec: do not read an unset MAC header in macsec_encrypt()"); other > TX .xmit paths that read eth_hdr(skb)->h_dest (several DSA taggers, > ibmveth, sja1105, the atlantic PTP path) have the same problem. > > packet_parse_headers() runs only on the transmit path > (packet_sendmsg_spkt(), tpacket_fill_skb(), packet_snd()), and there > skb->data is the start of the L2 header for every packet-socket type. There are some subtle points with variable length L2 protocols that need to be reviewed. > Reset the MAC header unconditionally so it is anchored for all of them, > fixing the class at the source rather than hardening each consumer. > > The protocol probe is unchanged. A CONFIG_DEBUG_NET build stops warning > about an unset mac header in skb_mac_header() on these paths, and the > out-of-bounds eth_hdr() read no longer occurs. > > 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 by user") > Cc: stable@vger.kernel.org > Assisted-by: 0sec:multi-model > Signed-off-by: Doruk Tan Ozturk > --- > This supersedes the per-consumer series "[PATCH net 0/3] net: dont read > an unset MAC header on the raw/qdisc-bypass TX path" > (https://lore.kernel.org/netdev/20260713194010.54642-1-doruk@0sec.ai/), > per Jakubs suggestion to fix the problem at the source rather than > hardening each driver. Vladimir Oltean had reviewed 2/3 of that series; > this is a different (source) fix, so I have not carried the tags. > net/packet/af_packet.c | 16 +++++++++++++--- > 1 file changed, 13 insertions(+), 3 deletions(-) > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c > index e75d2932475a..adfb9c19a3ca 100644 > --- a/net/packet/af_packet.c > +++ b/net/packet/af_packet.c > @@ -1924,11 +1924,21 @@ static void packet_parse_headers(struct sk_buff *skb, struct socket *sock) > { > int depth; > > + /* > + * packet_parse_headers() runs only on the transmit path > + * (packet_sendmsg_spkt(), tpacket_fill_skb(), packet_snd()), where > + * skb->data is the start of the L2 header for every packet-socket > + * type: SOCK_RAW and SOCK_PACKET carry a user-supplied header and > + * SOCK_DGRAM has one built by dev_hard_header(). Anchor the MAC > + * header for all of them so a frame does not reach ndo_start_xmit() > + * with the MAC header unset, where a driver reading eth_hdr(skb) on > + * TX would dereference an out-of-bounds offset (skb->head + (u16)~0). > + */ This is too verbose. Better captured in the commit message. > + skb_reset_mac_header(skb); > + > if ((!skb->protocol || skb->protocol == htons(ETH_P_ALL)) && > - sock->type == SOCK_RAW) { > - skb_reset_mac_header(skb); > + sock->type == SOCK_RAW) > skb->protocol = dev_parse_header_protocol(skb); > - } > > /* Move network header to the right position for VLAN tagged packets */ > if (likely(skb->dev->type == ARPHRD_ETHER) && > -- > 2.43.0 >