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[2a02:ab04:158:f000:2548:f3bd:8b42:b18f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4887a349fa1sm32485374f8f.8.2026.09.27.13.33.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 27 Sep 2026 13:33:29 -0700 (PDT) Date: Sun, 27 Sep 2026 16:33:27 -0400 From: "Michael S. Tsirkin" To: Eric Dumazet Cc: "David S . Miller" , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, edumazet@kernel.org, Weiming Shi , Willem de Bruijn , Jason Wang Subject: Re: [PATCH net] net: always dissect GSO packets in __virtio_net_hdr_to_skb() Message-ID: <20260927160750-mutt-send-email-mst@kernel.org> References: <20260927195536.2489079-1-edumazet@google.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260927195536.2489079-1-edumazet@google.com> On Sun, Sep 27, 2026 at 07:55:36PM +0000, Eric Dumazet wrote: > 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. > > However, skb->network_header is an offset from skb->head, not a boolean > flag. When skb_headroom(skb) is 0 on a device without L2 headers (for > instance packet_snd() or tpacket_snd() on a tunnel/pure-L3 device where > LL_RESERVED_SPACE_EX(dev, 0) == 0), skb_reset_network_header(skb) > legitimately sets skb->network_header to 0. > > 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 an IPv4 packet > carries IP options (ihl > 5, up to 60 bytes) or an IPv6 packet carries > extension headers, pulling only nh_min_len + thlen leaves the TCP header > outside skb->head, causing tcp_hdrlen(skb) in skb_gso_transport_seglen() > to read out-of-bounds: > > BUG: KASAN: slab-out-of-bounds in skb_gso_transport_seglen+0x173/0x200 net/core/skbuff.c:5503 > Read of size 1 at addr ffff888103c7ec4d by task repro/5718 > Call Trace: > > skb_gso_transport_seglen+0x173/0x200 net/core/skbuff.c:5503 > skb_gso_network_seglen include/linux/skbuff.h:4718 [inline] > skb_gso_validate_network_len+0x92/0x1a0 net/core/skbuff.c:5566 > ip_finish_output_gso net/ipv4/ip_output.c:285 [inline] > __ip_finish_output+0x28b/0x4a0 net/ipv4/ip_output.c:316 > > In addition, when skb->protocol is pre-set by a caller before > __virtio_net_hdr_to_skb(), 'if (!skb->protocol)' is skipped and > virtio_net_hdr_match_proto() was not checked. > > 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(). > 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. Validating virtio_net_hdr_match_proto(keys.basic.n_proto, hdr_gso_type) > after skb_flow_dissect_flow_keys_basic(). > > 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/ > Assisted-by: LLM > Signed-off-by: Eric Dumazet > Cc: Willem de Bruijn > Cc: Jason Wang > Cc: Michael S. Tsirkin > --- > arch/um/drivers/vector_transports.c | 1 + > drivers/net/tun.c | 23 ++++++---- > drivers/net/virtio_net.c | 2 + > include/linux/virtio_net.h | 66 ++++++++++++++--------------- > 4 files changed, 48 insertions(+), 44 deletions(-) > > diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c > index ddd127ee96785daa4c485b2a06f078686efd2046..e1fb2a76fdff59ef3032d091be06a06fa58046cc 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 5a302709a68aa3308b0c850b4a2957df3260b352..242899f7fd0711c5c59ff85accbf5bd1be9c6f39 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); Could you explain why is this being removed? What sets the mac header then? Thanks! > + 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 @@ static int tun_xdp_one(struct tun_struct *tun, > > 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 bf82ef9874abb4094931496fb6064a12ee75b789..daab43ac92ce4276b0f8f88684991e3993022ce6 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 c381b916c1b54afacba5473888af589a24fe087c..8902d5a5c418c46af0a6e7b26ae89948c20b0b54 100644 > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -111,48 +111,44 @@ 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; > - } > -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; > - } > + struct flow_keys_basic keys; > > - p_off = keys.control.thoff + thlen; > - if (!pskb_may_pull(skb, p_off) || > - keys.basic.ip_proto != ip_proto) > - return -EINVAL; > + if (!skb->protocol) { > + __be16 protocol = dev_parse_header_protocol(skb); > > - 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)) > + 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 || > + !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) { > -- > 2.56.0.rc1.315.gc6ed9934b7-goog