CVE-2026-89756: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()

CVE-2026-89756: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()

[Greg Kroah-Hartman]

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

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

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]

The Linux kernel CVE team has assigned CVE-2026-89756 to this issue.


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

	Fixed in 6.12.109 with commit 4996a7bc01ef35570664854dac2530c604981039
	Fixed in 6.18.50 with commit 5dc0daff0341c6baba19c38f47d299ac831d7e99
	Fixed in 7.2.4 with commit 66734981b4d3c105223a13c827c9c73be18d91ad
	Fixed in 7.3-rc1 with commit efe8f86c0916f0f74eea74ae21a3b37f728c6bad

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-2026-89756
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:
	mm/migrate.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/4996a7bc01ef35570664854dac2530c604981039
	https://git.kernel.org/stable/c/5dc0daff0341c6baba19c38f47d299ac831d7e99
	https://git.kernel.org/stable/c/66734981b4d3c105223a13c827c9c73be18d91ad
	https://git.kernel.org/stable/c/efe8f86c0916f0f74eea74ae21a3b37f728c6bad