CVE-2026-53250: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

CVE-2026-53250: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

[Greg Kroah-Hartman]

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

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

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

xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

The TX metadata area resides in the UMEM buffer which is memory-mapped
and concurrently writable by userspace. In xsk_skb_metadata(),
csum_start and csum_offset are read from shared memory for bounds
validation, then read again for skb assignment. A malicious userspace
application can race to overwrite these values between the two reads,
bypassing the bounds check and causing out-of-bounds memory access
during checksum computation in the transmit path.

Fix this by reading csum_start and csum_offset into local variables
once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launch_time) and the cached
csum fields may be mutually inconsistent due to concurrent userspace
writes, but this is benign: the only security-critical invariant is
that each field's validated value is the same one used, which local
caching guarantees.

The Linux kernel CVE team has assigned CVE-2026-53250 to this issue.


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

	Issue introduced in 6.8 with commit 48eb03dd26304c24f03bdbb9382e89c8564e71df and fixed in 6.18.36 with commit 0dfe05b938435892875e07771170051346412df9
	Issue introduced in 6.8 with commit 48eb03dd26304c24f03bdbb9382e89c8564e71df and fixed in 7.0.13 with commit bfdfd2706d5fb2cd496a1506e680daf979309c8b
	Issue introduced in 6.8 with commit 48eb03dd26304c24f03bdbb9382e89c8564e71df and fixed in 7.1 with commit 22ba97ea9cc1f63a0d0244fae38057ed452b6ac7

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-2026-53250
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/xdp/xsk.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/0dfe05b938435892875e07771170051346412df9
	https://git.kernel.org/stable/c/bfdfd2706d5fb2cd496a1506e680daf979309c8b
	https://git.kernel.org/stable/c/22ba97ea9cc1f63a0d0244fae38057ed452b6ac7