smp: Make CSD lock acquisition atomic for debug mode

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Summary

CVECVE-2026-68438
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-12 00:17:44 UTC
Updated2026-08-12 00:17:44 UTC
DescriptionIn 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.

Risk And Classification

EPSS: 0.001450000 probability, percentile 0.042690000 (date 2026-08-12)

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected b0473dcd4b1d7e2e44947e7ac1820c73a268821a 282d220bae5fbfc90cf0e3d5b5e42c00ad79f989 git Not specified
CNA Linux Linux affected b0473dcd4b1d7e2e44947e7ac1820c73a268821a 35551efb155e3b83445a6c3f66cb498d5efc182c git Not specified
CNA Linux Linux affected 7.1 Not specified
CNA Linux Linux unaffected 7.1 semver Not specified
CNA Linux Linux unaffected 7.1.6 7.1.* semver Not specified
CNA Linux Linux unaffected 7.2-rc5 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/282d220bae5fbfc90cf0e3d5b5e42c00ad79f989 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/35551efb155e3b83445a6c3f66cb498d5efc182c 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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