CVE-2024-43897

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

net: drop bad gso csum_start and offset in virtio_net_hdr

Tighten csum_start and csum_offset checks in virtio_net_hdr_to_skb for GSO packets.

The function already checks that a checksum requested with VIRTIO_NET_HDR_F_NEEDS_CSUM is in skb linear. But for GSO packets this might not hold for segs after segmentation.

Syzkaller demonstrated to reach this warning in skb_checksum_help

offset = skb_checksum_start_offset(skb);
ret = -EINVAL;
if (WARN_ON_ONCE(offset >= skb_headlen(skb)))

By injecting a TSO packet:

WARNING: CPU: 1 PID: 3539 at net/core/dev.c:3284 skb_checksum_help+0x3d0/0x5b0 ip_do_fragment+0x209/0x1b20 net/ipv4/ip_output.c:774 ip_finish_output_gso net/ipv4/ip_output.c:279 [inline] __ip_finish_output+0x2bd/0x4b0 net/ipv4/ip_output.c:301 iptunnel_xmit+0x50c/0x930 net/ipv4/ip_tunnel_core.c:82 ip_tunnel_xmit+0x2296/0x2c70 net/ipv4/ip_tunnel.c:813 __gre_xmit net/ipv4/ip_gre.c:469 [inline] ipgre_xmit+0x759/0xa60 net/ipv4/ip_gre.c:661 __netdev_start_xmit include/linux/netdevice.h:4850 [inline] netdev_start_xmit include/linux/netdevice.h:4864 [inline] xmit_one net/core/dev.c:3595 [inline] dev_hard_start_xmit+0x261/0x8c0 net/core/dev.c:3611 __dev_queue_xmit+0x1b97/0x3c90 net/core/dev.c:4261 packet_snd net/packet/af_packet.c:3073 [inline]

The geometry of the bad input packet at tcp_gso_segment:

[ 52.003050][ T8403] skb len=12202 headroom=244 headlen=12093 tailroom=0 [ 52.003050][ T8403] mac=(168,24) mac_len=24 net=(192,52) trans=244 [ 52.003050][ T8403] shinfo(txflags=0 nr_frags=1 gso(size=1552 type=3 segs=0)) [ 52.003050][ T8403] csum(0x60000c7 start=199 offset=1536 ip_summed=3 complete_sw=0 valid=0 level=0)

Mitigate with stricter input validation.

csum_offset: for GSO packets, deduce the correct value from gso_type. This is already done for USO. Extend it to TSO. Let UFO be: udp[46]_ufo_fragment ignores these fields and always computes the checksum in software.

csum_start: finding the real offset requires parsing to the transport header. Do not add a parser, use existing segmentation parsing. Thanks to SKB_GSO_DODGY, that also catches bad packets that are hw offloaded. Again test both TSO and USO. Do not test UFO for the above reason, and do not test UDP tunnel offload.

GSO packet are almost always CHECKSUM_PARTIAL. USO packets may be CHECKSUM_NONE since commit 10154dbded6d6 ("udp: Allow GSO transmit from devices with no checksum offload"), but then still these fields are initialized correctly in udp4_hwcsum/udp6_hwcsum_outgoing. So no need to test for ip_summed == CHECKSUM_PARTIAL first.

This revises an existing fix mentioned in the Fixes tag, which broke small packets with GSO offload, as detected by kselftests.

Red Hat rates the impact of this flaw as LOW. It is a vulnerability in the Linux kernel's virtio network header processing: the virtio_net_hdr_to_skb() function did not sufficiently validate the csum_start and csum_offset fields of GSO (for example TSO) packets, so a local attacker with low privileges able to inject a specially crafted GSO packet with invalid checksum metadata could cause the checksum start offset to fall outside the socket buffer linear data after segmentation, triggering a WARN_ON_ONCE in skb_checksum_help() during packet transmit. The demonstrated consequence is a kernel warning and potential denial of service limited to availability, as reflected in the Red Hat CVSS 3.1 score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).

In the Red Hat product portfolio, only the Fedora kernel (fedora-all) is currently listed as affected, and the fix is being pursued through the upstream community kernel process (DELEGATED). The currently tracked Red Hat Enterprise Linux kernel and kernel-rt streams and the Hosted OpenShift cluster environments (ARO and OSD) are marked as not affected by this issue.

The upstream CVE record assigns a higher severity, with a CVSS 3.1 score of 7.8 and confidentiality, integrity, and availability impact. Red Hat's assessment differs because the verified defect is limited to insufficient input validation of GSO checksum metadata that causes a kernel warning and availability impact, with no evidence of confidentiality or integrity impact.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.