From 99bda1ecbd56905f8bd934b9cdccd3bd251192b1 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 1 Oct 2026 19:11:38 +0000 Subject: [PATCH 1/3] flow_dissector: avoid u16 truncation of skb->len when computing thoff __skb_flow_dissect() computes key_control->thoff using: key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen); min_t(u16, ...) casts both arguments to u16 before comparing them. Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the truncated length wins and thoff is set to a bogus small value, even when the dissection succeeded. For example, an IPv4/TCP frame with skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as skb_probe_transport_header() point the transport header inside the Ethernet header. Such skbs are not exotic: - At the time of commit d0c081b49137 ("flow_dissector: properly cap thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO skbs larger than 64KB (MTU checks are skipped for GSO, and alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0 pages on top of the linear part). - BIG TCP now makes skbs larger than 64KB common. - The following patch makes tun_get_user() dissect IFF_TAP frames before eth_type_trans() pulls the Ethernet header, so a GSO frame carrying a 65522..65535 byte L3 packet will be dissected with skb->len in [65536, 65549]. Compare as u32 instead. If the resulting offset cannot be represented in the u16 key_control->thoff, cap it to U16_MAX and report the dissection as failed rather than silently returning a wrong transport offset. thoff is still set on failure, as some callers (such as eth_get_headlen()) use it regardless of the return value. Fixes: d0c081b49137 ("flow_dissector: properly cap thoff field") Signed-off-by: Eric Dumazet Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20261001191140.2818991-2-edumazet@kernel.org Signed-off-by: Jakub Kicinski --- net/core/flow_dissector.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c index 8aa4f9b4df81..27d8a01bc923 100644 --- a/net/core/flow_dissector.c +++ b/net/core/flow_dissector.c @@ -1071,6 +1071,7 @@ bool __skb_flow_dissect(const struct net *net, int mpls_lse = 0; int num_hdrs = 0; u8 ip_proto = 0; + u32 thoff; bool ret; if (!data) { @@ -1692,7 +1693,13 @@ out_good: ret = true; out: - key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen); + thoff = min_t(u32, nhoff, skb ? skb->len : hlen); + if (unlikely(thoff > U16_MAX)) { + /* Cannot be represented in key_control->thoff. */ + thoff = U16_MAX; + ret = false; + } + key_control->thoff = thoff; key_basic->n_proto = proto; key_basic->ip_proto = ip_proto; From 44378d02c5aaae24b60a75fbbb539b4aa4db503d Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 1 Oct 2026 19:11:39 +0000 Subject: [PATCH 2/3] 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 --- arch/um/drivers/vector_transports.c | 1 + drivers/net/tun.c | 23 +++++++---- drivers/net/virtio_net.c | 2 + include/linux/virtio_net.h | 64 ++++++++++++----------------- 4 files changed, 44 insertions(+), 46 deletions(-) diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c index ddd127ee9678..e1fb2a76fdff 100644 --- a/arch/um/drivers/vector_transports.c +++ b/arch/um/drivers/vector_transports.c @@ -209,6 +209,7 @@ static int raw_verify_header( if ((vheader->flags & VIRTIO_NET_HDR_F_DATA_VALID) > 0) return 1; + skb_set_network_header(skb, ETH_HLEN); virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian()); return 0; } diff --git a/drivers/net/tun.c b/drivers/net/tun.c index 5a302709a68a..242899f7fd07 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c @@ -1897,12 +1897,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, } } - if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) { - atomic_long_inc(&tun->rx_frame_errors); - err = -EINVAL; - goto free_skb; - } - + skb->dev = tun->dev; switch (tun->flags & TUN_TYPE_MASK) { case IFF_TUN: if (tun->flags & IFF_NO_PI) { @@ -1927,9 +1922,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, } } - skb_reset_mac_header(skb); + skb_reset_network_header(skb); skb->protocol = pi.proto; - skb->dev = tun->dev; break; case IFF_TAP: if (!pskb_may_pull(skb, ETH_HLEN)) { @@ -1937,10 +1931,19 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, drop_reason = SKB_DROP_REASON_HDR_TRUNC; goto drop; } - skb->protocol = eth_type_trans(skb, tun->dev); + skb_set_network_header(skb, ETH_HLEN); break; } + if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) { + atomic_long_inc(&tun->rx_frame_errors); + err = -EINVAL; + goto free_skb; + } + + if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP) + skb->protocol = eth_type_trans(skb, tun->dev); + /* copy skb_ubuf_info for callback when skb has no error */ if (zerocopy) { skb_zcopy_init(skb, msg_control); @@ -2600,6 +2603,8 @@ build: features = tun_vnet_hdr_guest_features(READ_ONCE(tun->vnet_hdr_sz)); tnl_hdr = (struct virtio_net_hdr_v1_hash_tunnel *)gso; + skb->dev = tun->dev; + skb_set_network_header(skb, ETH_HLEN); if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr)) { atomic_long_inc(&tun->rx_frame_errors); kfree_skb(skb); diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index bf82ef9874ab..daab43ac92ce 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -2515,6 +2515,8 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue * goto frame_err; } + skb->dev = dev; + skb_set_network_header(skb, ETH_HLEN); if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, vi->rx_tnl, vi->rx_tnl_csum, virtio_is_little_endian(vi->vdev))) { 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; + 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; - } -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; - } - - p_off = keys.control.thoff + thlen; - if (!pskb_may_pull(skb, p_off) || - keys.basic.ip_proto != ip_proto) - 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; + 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; + } + + 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); } if (hdr_gso_type != VIRTIO_NET_HDR_GSO_NONE) { From ee2d4c2d7cc18a2cd504044a75ea614e720248eb Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 1 Oct 2026 19:11:40 +0000 Subject: [PATCH 3/3] selftests: net: tun: add test for VLAN-tagged GSO without NEEDS_CSUM Add selftests in tun.c verifying that a VLAN-tagged (802.1Q) TCPv4 GSO packet without VIRTIO_NET_HDR_F_NEEDS_CSUM (both flags = 0 and flags = VIRTIO_NET_HDR_F_DATA_VALID) is accepted when written to a TAP device (/dev/net/tun with IFF_TAP | IFF_NO_PI | IFF_VNET_HDR). Also verify that the following are rejected with -EINVAL: - a mismatched GSO type (VIRTIO_NET_HDR_GSO_TCPV6 on a VLAN-tagged IPv4 packet). - a frame whose TCP header is truncated after 10 bytes. Pulling only sizeof(struct iphdr) + sizeof(struct tcphdr) bytes would accept it, so this requires the transport offset found by flow dissection. Finally, verify that a 65540-byte frame is accepted. Its skb->len is above U16_MAX while it is flow-dissected, before eth_type_trans() pulls the Ethernet header. Based on a reproducer by Michael S. Tsirkin . Signed-off-by: Eric Dumazet Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20261001191140.2818991-4-edumazet@kernel.org Signed-off-by: Jakub Kicinski --- tools/testing/selftests/net/tun.c | 136 ++++++++++++++++++++++++++++++ 1 file changed, 136 insertions(+) diff --git a/tools/testing/selftests/net/tun.c b/tools/testing/selftests/net/tun.c index abe488bac50b..afaa81c9bac5 100644 --- a/tools/testing/selftests/net/tun.c +++ b/tools/testing/selftests/net/tun.c @@ -9,6 +9,7 @@ #include #include #include +#include #include #include @@ -542,6 +543,141 @@ TEST_F(tun, reattach_close_delete) EXPECT_EQ(tun_delete(self->ifname), 0); } +FIXTURE(tun_vnet_gso) +{ + char ifname[IFNAMSIZ]; + int fd; +}; + +FIXTURE_SETUP(tun_vnet_gso) +{ + int flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR; + + memset(self->ifname, 0, sizeof(self->ifname)); + self->fd = tun_open(self->ifname, flags, 0, 0, NULL); + ASSERT_GE(self->fd, 0); +} + +FIXTURE_TEARDOWN(tun_vnet_gso) +{ + if (self->fd >= 0) + close(self->fd); +} + +static int build_vlan_tcpv4_gso_packet(uint8_t *buf, int payload_len) +{ + uint16_t vlan_tag[2] = { htons(100), htons(ETH_P_IP) }; + uint8_t *cur = buf + sizeof(struct virtio_net_hdr); + struct virtio_net_hdr vh = { 0 }; + struct tcphdr tcph = { 0 }; + uint32_t sum; + + cur += build_eth(cur, ETH_P_8021Q, param_hwaddr_outer_src, + param_hwaddr_outer_dst); + + /* 802.1Q tag: VID=100, inner protocol=ETH_P_IP */ + memcpy(cur, vlan_tag, sizeof(vlan_tag)); + cur += sizeof(vlan_tag); + + cur += build_ipv4_header(cur, IPPROTO_TCP, + sizeof(tcph) + payload_len, + ¶m_ipaddr4_outer_src, + ¶m_ipaddr4_outer_dst); + + tcph.source = htons(12345); + tcph.dest = htons(80); + tcph.seq = htonl(1); + tcph.doff = sizeof(tcph) / 4; + tcph.ack = 1; + tcph.window = htons(65535); + memcpy(cur, &tcph, sizeof(tcph)); + memset(cur + sizeof(tcph), PKT_DATA, payload_len); + + sum = add_csum((const uint8_t *)¶m_ipaddr4_outer_src, + sizeof(param_ipaddr4_outer_src)); + sum += add_csum((const uint8_t *)¶m_ipaddr4_outer_dst, + sizeof(param_ipaddr4_outer_dst)); + sum += htons(IPPROTO_TCP) + htons(sizeof(tcph) + payload_len); + sum += add_csum(cur, sizeof(tcph) + payload_len); + tcph.check = finish_ip_csum(sum); + memcpy(cur, &tcph, sizeof(tcph)); + cur += sizeof(tcph) + payload_len; + + vh.gso_type = VIRTIO_NET_HDR_GSO_TCPV4; + vh.gso_size = 1400; + vh.hdr_len = (cur - buf) - sizeof(vh) - payload_len; + memcpy(buf, &vh, sizeof(vh)); + + return cur - buf; +} + +TEST_F(tun_vnet_gso, vlan_tcpv4_gso_no_csum) +{ + struct virtio_net_hdr vh; + uint8_t pkt[4096] = { 0 }; + int len, ret; + + len = build_vlan_tcpv4_gso_packet(pkt, 2800); + memcpy(&vh, pkt, sizeof(vh)); + + /* Valid VLAN-tagged TCPv4 GSO with flags = 0 (no NEEDS_CSUM) */ + vh.flags = 0; + memcpy(pkt, &vh, sizeof(vh)); + ret = write(self->fd, pkt, len); + ASSERT_EQ(ret, len); + + /* Valid VLAN-tagged TCPv4 GSO with flags = DATA_VALID */ + vh.flags = VIRTIO_NET_HDR_F_DATA_VALID; + memcpy(pkt, &vh, sizeof(vh)); + ret = write(self->fd, pkt, len); + ASSERT_EQ(ret, len); + + /* Mismatched GSO type (TCPV6 on VLAN-tagged IPv4 packet) */ + vh.flags = 0; + vh.gso_type = VIRTIO_NET_HDR_GSO_TCPV6; + memcpy(pkt, &vh, sizeof(vh)); + ret = write(self->fd, pkt, len); + ASSERT_EQ(ret, -1); + ASSERT_EQ(errno, EINVAL); + + /* TCP header truncated after 10 bytes, without NEEDS_CSUM. + * Requires the transport header offset found by flow dissection: + * pulling only sizeof(struct iphdr) + sizeof(struct tcphdr) bytes + * from the mac header would accept this frame. + */ + vh.gso_type = VIRTIO_NET_HDR_GSO_TCPV4; + vh.hdr_len = ETH_HLEN + 4 + sizeof(struct iphdr) + 10; + memcpy(pkt, &vh, sizeof(vh)); + ret = write(self->fd, pkt, sizeof(vh) + vh.hdr_len); + ASSERT_EQ(ret, -1); + ASSERT_EQ(errno, EINVAL); +} + +TEST_F(tun_vnet_gso, vlan_tcpv4_gso_no_csum_64k) +{ + /* Ethernet frame of 65540 bytes: skb->len is above U16_MAX when the + * frame is flow-dissected, before eth_type_trans() pulls ETH_HLEN. + */ + const int payload_len = 65540 - ETH_HLEN - 4 - + sizeof(struct iphdr) - sizeof(struct tcphdr); + struct virtio_net_hdr vh; + uint8_t *pkt; + int len, ret; + + pkt = calloc(1, sizeof(vh) + 65540); + ASSERT_NE(pkt, NULL); + + len = build_vlan_tcpv4_gso_packet(pkt, payload_len); + ASSERT_EQ(len, sizeof(vh) + 65540); + memcpy(&vh, pkt, sizeof(vh)); + + vh.flags = 0; + memcpy(pkt, &vh, sizeof(vh)); + ret = write(self->fd, pkt, len); + free(pkt); + ASSERT_EQ(ret, len); +} + FIXTURE(tun_vnet_udptnl) { char ifname[IFNAMSIZ];