From 44378d02c5aaae24b60a75fbbb539b4aa4db503d Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 1 Oct 2026 19:11:39 +0000 Subject: 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. Fixes: 9e8db5913264 ("net: avoid false positives in untrusted gso validation") Fixes: d5be7f632bad ("net: validate untrusted gso packets without csum offload") Fixes: 924a9bc362a5 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct") Reported-by: Weiming Shi Closes: https://lore.kernel.org/netdev/20260927163117.746432-2-bestswngs@gmail.com/ Signed-off-by: Eric Dumazet Reviewed-by: Willem de Bruijn Cc: Michael S. Tsirkin Link: https://patch.msgid.link/20261001191140.2818991-3-edumazet@kernel.org Signed-off-by: Jakub Kicinski --- include/linux/virtio_net.h | 60 +++++++++++++++++++--------------------------- 1 file changed, 25 insertions(+), 35 deletions(-) (limited to 'include') diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h index c381b916c1b5..d6466f96cdd0 100644 --- a/include/linux/virtio_net.h +++ b/include/linux/virtio_net.h @@ -111,48 +111,38 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb, p_off = nh_min_len + thlen; if (!pskb_may_pull(skb, p_off)) return -EINVAL; - } else { + } else if (gso_type) { /* gso packets without NEEDS_CSUM do not set transport_offset. * probe and drop if does not match one of the above types. */ - if (gso_type && skb->network_header) { - struct flow_keys_basic keys; - - if (!skb->protocol) { - __be16 protocol = dev_parse_header_protocol(skb); - - if (!protocol) - virtio_net_hdr_set_proto(skb, hdr); - else if (!virtio_net_hdr_match_proto(protocol, - hdr_gso_type)) - return -EINVAL; - else - skb->protocol = protocol; - } + struct flow_keys_basic keys; + + if (!skb->protocol) { + skb->protocol = dev_parse_header_protocol(skb); + if (!skb->protocol) + virtio_net_hdr_set_proto(skb, hdr); + } retry: - if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys, - NULL, 0, 0, 0, - 0)) { - /* UFO does not specify ipv4 or 6: try both */ - if (gso_type & SKB_GSO_UDP && - skb->protocol == htons(ETH_P_IP)) { - skb->protocol = htons(ETH_P_IPV6); - goto retry; - } - return -EINVAL; + if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys, + NULL, 0, 0, 0, + 0)) { + /* UFO does not specify ipv4 or 6: try both */ + if (gso_type & SKB_GSO_UDP && + skb->protocol == htons(ETH_P_IP)) { + skb->protocol = htons(ETH_P_IPV6); + goto retry; } + return -EINVAL; + } - p_off = keys.control.thoff + thlen; - if (!pskb_may_pull(skb, p_off) || - keys.basic.ip_proto != ip_proto) - return -EINVAL; + p_off = keys.control.thoff + thlen; + if (!pskb_may_pull(skb, p_off) || + keys.basic.ip_proto != ip_proto || + !virtio_net_hdr_match_proto(keys.basic.n_proto, + hdr_gso_type)) + return -EINVAL; - skb_set_transport_header(skb, keys.control.thoff); - } else if (gso_type) { - p_off = nh_min_len + thlen; - if (!pskb_may_pull(skb, p_off)) - return -EINVAL; - } + skb_set_transport_header(skb, keys.control.thoff); } if (hdr_gso_type != VIRTIO_NET_HDR_GSO_NONE) { -- cgit v1.2.3