CVE-2026-74695: netfilter: nf_flow_table: drop existing skb dst before skb_dst_set_noref()

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

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

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

netfilter: nf_flow_table: drop existing skb dst before skb_dst_set_noref()

Incoming skbs passing through netfilter flowtable offload hooks (or XFRM
offload path) might already carry a ref-counted dst_entry assigned during
earlier RX or routing steps.

Calling skb_dst_set_noref() when skb already holds a ref-counted dst
overwrites skb->_skb_refdst, leaking the previous dst_entry reference
count and triggering a DEBUG_NET_WARN_ON_ONCE assertion in
skb_dst_check_unset():

  WARNING: at skb_dst_check_unset include/linux/skbuff.h:1170
  WARNING: at skb_dst_set_noref include/linux/skbuff.h:1234
  WARNING: at nf_flow_offload_ip_hook+0xf6c/0x2b60 net/netfilter/nf_flow_table_ip.c:864

Drop any existing dst_entry reference with skb_dst_drop(skb) before
setting the non-referenced flowtable destination.

The Linux kernel CVE team has assigned CVE-2026-74695 to this issue.


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

	Issue introduced in 4.18 with commit 2a79fd3908acd88e6cb0e620c314d7b1fee56a02 and fixed in 6.18.45 with commit 12afa450a6a6c0cce2c42b7545a9958f62d8a00c
	Issue introduced in 4.18 with commit 2a79fd3908acd88e6cb0e620c314d7b1fee56a02 and fixed in 7.1.9 with commit 538e67e8c7889cf5f93951f5309d1bcb41f86036
	Issue introduced in 4.18 with commit 2a79fd3908acd88e6cb0e620c314d7b1fee56a02 and fixed in 7.2 with commit 8aecf0bbcc72605592134c917c222207d8f63ab0
	Issue introduced in 4.16.15 with commit b4b1adf2e66ecc7125c4117e7aad9ff61e2cfd27
	Issue introduced in 4.17.1 with commit 209dedf806d31095968f54323dbe62525b077b33

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-2026-74695
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/netfilter/nf_flow_table_ip.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/12afa450a6a6c0cce2c42b7545a9958f62d8a00c
	https://git.kernel.org/stable/c/538e67e8c7889cf5f93951f5309d1bcb41f86036
	https://git.kernel.org/stable/c/8aecf0bbcc72605592134c917c222207d8f63ab0