CVE-2026-74665: net: fix skb length accounting after generic XDP frag adjustment

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

net: fix skb length accounting after generic XDP frag adjustment

Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

The Linux kernel CVE team has assigned CVE-2026-74665 to this issue.


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

	Issue introduced in 6.9 with commit e6d5dbdd20aa6a86974af51deb9414cd2e7794cb and fixed in 6.12.104 with commit 72468efcc089782fa047afddd10856b11a14ef5b
	Issue introduced in 6.9 with commit e6d5dbdd20aa6a86974af51deb9414cd2e7794cb and fixed in 6.18.45 with commit 5f30f9c302cea1f2ebf97abe574c1826544223d7
	Issue introduced in 6.9 with commit e6d5dbdd20aa6a86974af51deb9414cd2e7794cb and fixed in 7.1.9 with commit ea1ccd6d1c6370868e3b032a3fc575b9c397f2e1
	Issue introduced in 6.9 with commit e6d5dbdd20aa6a86974af51deb9414cd2e7794cb and fixed in 7.2 with commit 33f2b2eb33d666ecac68031e0f31424fb70528db

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-2026-74665
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:
	net/core/dev.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/72468efcc089782fa047afddd10856b11a14ef5b
	https://git.kernel.org/stable/c/5f30f9c302cea1f2ebf97abe574c1826544223d7
	https://git.kernel.org/stable/c/ea1ccd6d1c6370868e3b032a3fc575b9c397f2e1
	https://git.kernel.org/stable/c/33f2b2eb33d666ecac68031e0f31424fb70528db