Subject: CVE-2024-43897: net: drop bad gso csum_start and offset in virtio_net_hdr

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2024-43897 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.15.165 with commit 27874ca77bd2 and fixed in 5.15.167 with commit 413e785a89f8
	Issue introduced in 6.1.103 with commit 5b1997487a3f and fixed in 6.1.107 with commit f01c5e335fbb
	Issue introduced in 6.6.44 with commit 90d41ebe0cd4 and fixed in 6.6.46 with commit 6772c4868a8e
	Issue introduced in 6.10.3 with commit e9164903b8b3 and fixed in 6.10.5 with commit 2edbb3e8838c

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-43897
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	include/linux/virtio_net.h
	net/ipv4/tcp_offload.c
	net/ipv4/udp_offload.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/413e785a89f8bde0d4156a54b8ac2fa003c06756
	https://git.kernel.org/stable/c/f01c5e335fbb7fb612d40f14a3c02e2612a43d3b
	https://git.kernel.org/stable/c/6772c4868a8e7ad5305957cdb834ce881793acb7
	https://git.kernel.org/stable/c/2edbb3e8838c672cd7e247e47989df9d03fc6668
	https://git.kernel.org/stable/c/89add40066f9ed9abe5f7f886fe5789ff7e0c50e