net: always dissect GSO packets in __virtio_net_hdr_to_skb()

Overview

In the Linux kernel, the following vulnerability has been resolved:

net: always dissect GSO packets in __virtio_net_hdr_to_skb()

Commit 9e8db5913264 ("net: avoid false positives in untrusted gso validation") added a '&& skb->network_header' check before flow-dissecting GSO packets without VIRTIO_NET_HDR_F_NEEDS_CSUM in _virtio_net_hdr_to_skb(), because some callers (such as tun_get_user(), tun_xdp_one(), virtnet_receive_done(), and raw_verify_header()) called virtio_net_hdr*_to_skb() before initializing skb->network_header and skb->dev.

Because __alloc_skb() and __build_skb_around() zero-initialize skb->network_header to 0 (unlike mac_header and transport_header which are initialized to ~0U), those four callers always had skb->network_header == 0 and bypassed flow dissection in __virtio_net_hdr_to_skb(). More generally, skb->network_header is an offset from skb->head (where 0 is also a valid offset whenever skb_headroom(skb) is 0), not a boolean flag.

Whenever the 'if (gso_type && skb->network_header)' branch was skipped, the fallback 'else if (gso_type)' only pulled nh_min_len + thlen (40 bytes for TCPv4) without dissecting the packet, without validating ip_proto or n_proto, and without setting skb->transport_header.

If the packet has a malformed network header, it is not rejected and a subsequent skb_probe_transport_header() also fails, leaving skb->transport_header at ~0U (0xffff). Similarly, if an IPv4 packet carries IP options (ihl > 5) or an IPv6 packet carries extension headers, pulling only nh_min_len + thlen can leave the TCP header outside skb->head. In both cases, tcp_hdrlen(skb) in skb_gso_transport_seglen() reads out-of-bounds:

BUG: KASAN: slab-out-of-bounds in skb_gso_transport_seglen Read of size 2 by task poc/133 skb_gso_transport_seglen (net/core/gso.c:155) skb_gso_validate_mac_len (net/core/gso.c:270) tbf_enqueue (net/sched/sch_tbf.c:260) dev_qdisc_enqueue (net/core/dev.c:4227) __dev_queue_xmit (net/core/dev.c:4884)

In addition, checking virtio_net_hdr_match_proto() only inside 'if (!skb->protocol)' before flow dissection both skipped validation when skb->protocol was pre-set by the caller and rejected VLAN-tagged frames whose outer L2 protocol is ETH_P_8021Q or ETH_P_8021AD.

Fix this by:

1. Initializing skb->dev and skb->network_header (plus skb->protocol for IFF_TUN) before virtio_net_hdr_*_to_skb() in tun_get_user(), tun_xdp_one(), virtnet_receive_done(), and raw_verify_header(). In tun_get_user(), drop the redundant skb_reset_mac_header(skb) in the IFF_TUN case since __virtio_net_hdr_to_skb() unconditionally resets mac_header.
2. Removing '&& skb->network_header' and the unvalidated 'else if (gso_type)' fallback in __virtio_net_hdr_to_skb() so all GSO packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are flow-dissected, have their transport header pulled into linear data, and have skb->transport_header set.
3. Moving the virtio_net_hdr_match_proto() check to after skb_flow_dissect_flow_keys_basic(), validating keys.basic.n_proto against hdr_gso_type.

Affected

• Linux >= d5be7f632bad0f489879eed0ff4b99bd7fe0b74c < 5f4dfdf0077730e9d3ef5b37ceb17a5a56d6f34c
• Linux >= d5be7f632bad0f489879eed0ff4b99bd7fe0b74c < 44378d02c5aaae24b60a75fbbb539b4aa4db503d
• Linux dac7d4432b13d6b58923c1178b786bcda37c9e9e
• Linux e93384b12443c9bb6830b56b7d299beed432b865
• Linux f709f7b28d76de327373598a0fda9f916c36b5e6
• Linux d996573ebd5c528c613e90b7e7daa961d58fa16e
• Linux c375152be9dd9d6fbf6ae5ac8be337d0590f192a
• Linux 0dab7eb86ea06ce8c530785f1999948c6f223c96
• Linux ea3fb2ce5fa794d02135f5c079e05cd6fc3f545d
• Linux 54ef8243c3c8e90f1ea5792e6752e021a25c8eb3
• Linux >= 4.14.104 < 4.15
• Linux >= 4.19.26 < 4.20
• Linux >= 4.20.13 < 4.21
• Linux >= 4.14.226 < 4.15
• Linux >= 4.19.181 < 4.20
• Linux 5.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.