CVE-2026-89756

Summary

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

mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()

migrate_pages_batch() unmaps each folio before moving it, and every unmap runs the mmu_notifier invalidate callbacks. On KVM hosts try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() -> tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps the CPU busy for a long time.

The loop already calls cond_resched(), but on PREEMPTION kernels that is a no-op, and involuntary preemption is not a Tasks-RCU quiescent state.

A long batch therefore never reports a quiescent state, and the migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the Tasks-RCU grace period for minutes, which is common at Meta fleet:

INFO: rcu_tasks detected stalls on tasks: 0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0 state:R running task Call Trace: tdp_mmu_zap_leafs tdp_mmu_next_root gfn_to_pfn_cache_invalidate_start kvm_mmu_notifier_invalidate_range_start __mmu_notifier_invalidate_range_start try_to_migrate_one try_to_migrate migrate_pages_batch migrate_pages compact_zone compact_node kcompactd kthread

Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even when cond_resched() does nothing.

This has also been discussed at [1]

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/4996a7bc01ef35570664854dac2530c604981039
• https://git.kernel.org/stable/c/5dc0daff0341c6baba19c38f47d299ac831d7e99
• https://git.kernel.org/stable/c/66734981b4d3c105223a13c827c9c73be18d91ad
• https://git.kernel.org/stable/c/efe8f86c0916f0f74eea74ae21a3b37f728c6bad

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