CVE-2026-68438: smp: Make CSD lock acquisition atomic for debug mode

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

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

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

smp: Make CSD lock acquisition atomic for debug mode

Commit b0473dcd4b1d ("smp: Improve smp_call_function_single()
CSD-lock diagnostics") changed smp_call_function_single() so that,
when CSD lock debugging is enabled, async !wait calls use the
destination CPU csd_data. That improves diagnostics, but it also removes
the single-writer property that made the old csd_lock() safe: multiple
CPUs can now prepare the same destination CPU CSD concurrently.

csd_lock() currently waits for CSD_FLAG_LOCK to clear and then sets the
bit with a non-atomic read-modify-write. Two senders can both see an
unlocked CSD, set the bit, overwrite the callback fields, and enqueue
the same llist node. Re-adding a node that is already the queue head can
make node->next point to itself, leaving the target CPU stuck walking
call_single_queue. Later synchronous work, such as a TLB shootdown, can
then remain queued and trigger soft-lockup warnings or panics.

Keep the single csd_lock() implementation, but when CSD lock debugging is
enabled, acquire CSD_FLAG_LOCK with try_cmpxchg_acquire(). This makes the
destination CPU CSD a real atomic lock in the only configuration where it
can be shared by multiple remote senders, while preserving the existing
non-debug fast path.

The Linux kernel CVE team has assigned CVE-2026-68438 to this issue.


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

	Issue introduced in 7.1 with commit b0473dcd4b1d7e2e44947e7ac1820c73a268821a and fixed in 7.1.6 with commit 282d220bae5fbfc90cf0e3d5b5e42c00ad79f989
	Issue introduced in 7.1 with commit b0473dcd4b1d7e2e44947e7ac1820c73a268821a and fixed in 7.2-rc5 with commit 35551efb155e3b83445a6c3f66cb498d5efc182c

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-2026-68438
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:
	kernel/smp.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/282d220bae5fbfc90cf0e3d5b5e42c00ad79f989
	https://git.kernel.org/stable/c/35551efb155e3b83445a6c3f66cb498d5efc182c