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Thu, 29 Oct 2026 21:22:39 -0700 (PDT) Received: from com-75606.corp.openai.org ([199.47.143.5]) by smtp.gmail.com with ESMTPSA id 5614622812f47-5640fe45587sm703285b6e.15.2026.10.29.21.22.39 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Thu, 29 Oct 2026 21:22:39 -0700 (PDT) From: Kyle Zeng To: netdev@vger.kernel.org Cc: Sabrina Dubroca , linux-kernel@vger.kernel.org, outbounddisclosures@openai.com Subject: [PATCH] macsec: reject frames that decapsulate into runt Ethernet frames Date: Thu, 29 Oct 2026 21:22:37 -0700 Message-ID: <20261030042237.74106-1-kylebot@openai.com> X-Mailer: git-send-email 2.55.0 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When an SCI is unknown and the C bit is clear, macsec_handle_frame() removes the SecTAG and the default ICV without checking how much of the frame remains. A short frame can therefore become an skb shorter than ETH_HLEN. eth_type_trans() cannot pull its Ethernet header, and a second stacked MACsec device can copy stale skb headroom into a packet delivered to a packet socket. The matched-SCI path has a related hole: macsec_validate_skb() accepts a short_length of one, so a frame that passes the existing length check can still become a 13-byte Ethernet frame. Require enough bytes for the MACsec overhead and a complete Ethernet header before decapsulating an unknown-SCI frame. Apply the same bound in macsec_validate_skb(), using the selected SecY's ICV length, to cover the no-SA, synchronous-decrypt and asynchronous-decrypt delivery paths. Leave other unknown-SCI handling unchanged. Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver") Cc: stable@vger.kernel.org Signed-off-by: Kyle Zeng --- I have a reproducer for this issue and can share via DM drivers/net/macsec.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c index 87f3f4bdb026..240ba0ae07d1 100644 --- a/drivers/net/macsec.c +++ b/drivers/net/macsec.c @@ -408,8 +408,10 @@ static bool macsec_validate_skb(struct sk_buff *skb, u16 icv_len, bool xpn) int len = skb->len - 2 * ETH_ALEN; int extra_len = macsec_extra_len(!!(h->tci_an & MACSEC_TCI_SC)) + icv_len; - /* a) It comprises at least 17 octets */ - if (skb->len <= 16) + /* a) It comprises at least 17 octets. The SecTAG and ICV must + * also leave a complete Ethernet header. + */ + if (skb->len < ETH_HLEN + extra_len) return false; /* b) MACsec EtherType: already checked */ @@ -1331,9 +1333,12 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb) nosci: /* 10.6.1 if the SC is not found */ cbit = !!(hdr->tci_an & MACSEC_TCI_C); - if (!cbit) - macsec_finalize_skb(skb, MACSEC_DEFAULT_ICV_LEN, - macsec_extra_len(macsec_skb_cb(skb)->has_sci)); + if (!cbit) { + len = macsec_extra_len(macsec_skb_cb(skb)->has_sci); + if (skb->len < ETH_HLEN + len + MACSEC_DEFAULT_ICV_LEN) + goto drop_nosc; + macsec_finalize_skb(skb, MACSEC_DEFAULT_ICV_LEN, len); + } list_for_each_entry_rcu(macsec, &rxd->secys, secys) { struct sk_buff *nskb; base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3 -- 2.56.0