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          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM\n\nWhen performing Generic Segmentation Offload (GSO) on an IPv6 packet that\ncontains extension headers, the kernel incorrectly requests checksum offload\nif the egress device only advertises NETIF_F_IPV6_CSUM feature, which has\na strict contract: it supports checksum offload only for plain TCP or UDP\nover IPv6 and explicitly does not support packets with extension headers.\nThe current GSO logic violates this contract by failing to disable the feature\nfor packets with extension headers, such as those used in GREoIPv6 tunnels.\n\nThis violation results in the device being asked to perform an operation\nit cannot support, leading to a `skb_warn_bad_offload` warning and a collapse\nof network throughput. While device TSO/USO is correctly bypassed in favor\nof software GSO for these packets, the GSO stack must be explicitly told not\nto request checksum offload.\n\nMask NETIF_F_IPV6_CSUM, NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4\nin gso_features_check if the IPv6 header contains extension headers to compute\nchecksum in software.\n\nThe exception is a BIG TCP extension, which, as stated in commit\n68e068cabd2c6c53 (\"net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets\"):\n\"The feature is only enabled on devices that support BIG TCP TSO.\nThe header is only present for PF_PACKET taps like tcpdump,\nand not transmitted by physical devices.\"\n\nkernel log output (truncated):\nWARNING: CPU: 1 PID: 5273 at net/core/dev.c:3535 skb_warn_bad_offload+0x81/0x140\n...\nCall Trace:\n \u003cTASK\u003e\n skb_checksum_help+0x12a/0x1f0\n validate_xmit_skb+0x1a3/0x2d0\n validate_xmit_skb_list+0x4f/0x80\n sch_direct_xmit+0x1a2/0x380\n __dev_xmit_skb+0x242/0x670\n __dev_queue_xmit+0x3fc/0x7f0\n ip6_finish_output2+0x25e/0x5d0\n ip6_finish_output+0x1fc/0x3f0\n ip6_tnl_xmit+0x608/0xc00 [ip6_tunnel]\n ip6gre_tunnel_xmit+0x1c0/0x390 [ip6_gre]\n dev_hard_start_xmit+0x63/0x1c0\n __dev_queue_xmit+0x6d0/0x7f0\n ip6_finish_output2+0x214/0x5d0\n ip6_finish_output+0x1fc/0x3f0\n ip6_xmit+0x2ca/0x6f0\n ip6_finish_output+0x1fc/0x3f0\n ip6_xmit+0x2ca/0x6f0\n inet6_csk_xmit+0xeb/0x150\n __tcp_transmit_skb+0x555/0xa80\n tcp_write_xmit+0x32a/0xe90\n tcp_sendmsg_locked+0x437/0x1110\n tcp_sendmsg+0x2f/0x50\n...\nskb linear:   00000000: e4 3d 1a 7d ec 30 e4 3d 1a 7e 5d 90 86 dd 60 0e\nskb linear:   00000010: 00 0a 1b 34 3c 40 20 11 00 00 00 00 00 00 00 00\nskb linear:   00000020: 00 00 00 00 00 12 20 11 00 00 00 00 00 00 00 00\nskb linear:   00000030: 00 00 00 00 00 11 2f 00 04 01 04 01 01 00 00 00\nskb linear:   00000040: 86 dd 60 0e 00 0a 1b 00 06 40 20 23 00 00 00 00\nskb linear:   00000050: 00 00 00 00 00 00 00 00 00 12 20 23 00 00 00 00\nskb linear:   00000060: 00 00 00 00 00 00 00 00 00 11 bf 96 14 51 13 f9\nskb linear:   00000070: ae 27 a0 a8 2b e3 80 18 00 40 5b 6f 00 00 01 01\nskb linear:   00000080: 08 0a 42 d4 50 d5 4b 70 f8 1a"
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            "baseSeverity": "HIGH",
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              "value": "AV:N - The defect is in the generic packet-transmit path (`net/core/dev.c`), and the reproducer is a remote TCP peer whose connection to a service egressing over an ip6gre/ip6ip6 tunnel makes the kernel build a GSO skb carrying the default encap-limit destination-options header, dropping every reply and emitting a WARN. No local access is needed to drive the trigger.\nAC:L - The failure is fully deterministic \u2014 there is no race, timing window, or memory-layout dependency; `skb_csum_hwoffload_help()` unconditionally routes any GSO IPv6 skb with an extension header into `skb_checksum_help()`, which warns and drops on every single packet. The NIC feature set (`IPV6_CSUM` + `TSO6`, no `HW_CSUM`) is the common case across dozens of mainstream drivers.\nPR:N - The remote client needs no credentials \u2014 merely establishing a connection and pulling more than one MSS of data from a service behind an IPv6 tunnel is sufficient. Locally the bar is likewise nil: `ipv6_set_opt_hdr()` at `net/ipv6/ipv6_sockglue.c:312` exempts `IPV6_RTHDR` from its `CAP_NET_RAW` check, so any unprivileged process can attach an SRH to a TCPv6 socket and hit the same path.\nUI:N - The attacker\u0027s own traffic drives the transmit path directly; no administrator or victim needs to open a file, mount anything, or take any action.\nS:U - The WARN, the packet drop, and any resulting panic all occur inside the kernel\u0027s own security authority on the same host, crossing no VM, IOMMU, container, or sandbox boundary.\nC:N - The skb is discarded via `kfree_skb()` with no out-of-bounds access and no kernel memory read primitive; the attacker learns nothing, as `skb_dump()` output goes only to the target\u0027s ring buffer, which they cannot read.\nI:N - No data is modified \u2014 the packet is rejected before reaching the device rather than being transmitted with a bad checksum, and there is no out-of-bounds or freed-memory write anywhere on the path.\nA:H - `skb_warn_bad_offload()` fires a `WARN(1)` that taints the kernel and becomes a full panic under `panic_on_warn`, and `validate_xmit_skb()` then destroys the skb, so every qualifying packet is silently discarded \u2014 a total collapse of throughput and loss of connectivity for the affected tunnel or interface, sustained for as long as the attacker keeps sending."
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