CVE-2026-74558: xsk: reclaim invalid Tx descriptors in ZC batch path

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

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

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

xsk: reclaim invalid Tx descriptors in ZC batch path

The zero-copy Tx batch parser stops when it encounters an invalid
descriptor. If this happens after one or more continuation descriptors,
the Tx consumer can be advanced past fragments that are neither submitted
to the driver nor returned to userspace through the completion ring.

A similar problem occurs when a packet exceeds xdp_zc_max_segs. The
descriptors consumed up to the limit are released without completion, and
the remaining continuation descriptors can subsequently be interpreted
as the beginning of another packet.

Parse Tx batches in packet units and distinguish descriptors belonging to
complete valid packets from descriptors consumed while draining an
invalid or oversized packet. Return the former to the driver and append
the latter to the CQ address area so userspace can reclaim their UMEM
frames.

Treat a standalone invalid descriptor as a one-descriptor reclaim-only
packet. Advancing the Tx-ring consumer releases the ring slot, but does
not by itself return ownership of the referenced UMEM frame to userspace.

Once draining starts, continue until the packet's end-of-packet
descriptor is consumed. Preserve the drain state on the socket when EOP
has not yet been supplied, so draining can continue during a later call.
Leave incomplete but otherwise valid packets on the Tx ring.

Shared-UMEM pools using multi-buffer Tx also need packet-framed parsing.
Walk their Tx sockets one packet at a time, preserving the existing
per-socket fairness scheme, instead of using the legacy one-descriptor
fallback. Keep that fallback for shared pools that do not use
multi-buffer Tx. Since the drain state is maintained per socket and both
the singular and shared paths can resume an interrupted drain, changing
the socket list from singular to shared requires no special bind-time
transition.

CQ entries are positional, and drivers may complete only part of the Tx
work returned by xsk_tx_peek_release_desc_batch(). Therefore, reclaim-only
entries cannot be published immediately when earlier driver-visible
descriptors are still outstanding.

Track the number of driver-visible CQ entries preceding the reclaim
entries. Let xsk_tx_completed() publish partial hardware Tx completions,
and publish the reclaim entries only after every earlier Tx descriptor
has completed. Complete a reclaim-only batch immediately when there is no
driver-visible work in front of it, and prevent another Tx batch from
being appended while reclaim entries remain pending.

Also cap batch processing by the size of the pool's temporary descriptor
array, as Tx rings belonging to sockets sharing a UMEM may have different
sizes.

This ensures that every invalid Tx descriptor consumed by the ZC batch
path is either submitted to the driver as part of a valid packet or
returned to userspace without violating CQ completion ordering.

The Linux kernel CVE team has assigned CVE-2026-74558 to this issue.


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

	Issue introduced in 6.6 with commit cf24f5a5feeaae34c1a34d1e04f8ac697290427a and fixed in 7.1.8 with commit e7ce2bad0c33471c8da4dd73dff2119a68f96f3a
	Issue introduced in 6.6 with commit cf24f5a5feeaae34c1a34d1e04f8ac697290427a and fixed in 7.2-rc6 with commit 72f2b4516faf55d4dfac2414649d3cffa5fd2c5e

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-2026-74558
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:
	Documentation/networking/af_xdp.rst
	include/net/xsk_buff_pool.h
	net/xdp/xsk.c
	net/xdp/xsk_buff_pool.c
	net/xdp/xsk_queue.h


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/e7ce2bad0c33471c8da4dd73dff2119a68f96f3a
	https://git.kernel.org/stable/c/72f2b4516faf55d4dfac2414649d3cffa5fd2c5e