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

KVM: arm64: Fix spurious warning for benign stage 2 teardown race

kvmtool was used to establish an L1 guest with 8 CPUs and 8 GiB of RAM, an
L2 guest with 4 CPUs and 4 GiB of RAM and an L3 guest with 2 CPUs and 2 GiB
of RAM, all of which was then exited.

Under memory pressure in the L0 host warnings were observed due to
migration triggered by compaction:

WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80,
CPU#5: kcompactd0/66

Which was, in turn, triggered by an MMU notifier for the host invalidation:

mmu_notifier_invalidate_range_start()
-> ... -> kvm_mmu_notifier_invalidate_range_start()
-> kvm_mmu_unmap_gfn_range()
-> kvm_unmap_gfn_range()
-> kvm_nested_s2_unmap()
-> kvm_stage2_unmap_range()
-> __unmap_stage2_range()
-> stage2_apply_range()
<- -EINVAL, triggering a WARN_ON()

Racing with L0's teardown of stage 2 page tables:

exit_mm()
-> mmput()
-> __mmput()
-> exit_mmap()
-> mmu_notifier_release()
-> ... -> kvm_mmu_notifier_release()
-> kvm_flush_shadow_all()
-> kvm_arch_flush_shadow_all()
-> kvm_free_stage2_pgd()
-> [ acquire kvm->mmu_lock for write ]
-> mmu->pgt = NULL [ among other tasks ]
-> [ release kvm->mmu_lock for write ]

It turns out there is a benign race resulting in a spurious warning:

Thread A - notify: migration | Thread B - notify: release
-------------------------------|---------------------------------
< kvm->mmu_lock held > |
stage2_apply_range() |
get mmu->pgt, check !NULL |
... | kvm_arch_flush_shadow_all()
cond_resched_rwlock_write(); | < contend, sleep kvm->mmu_lock >
< drop kvm->mmu_lock > | < acquire kvm->mmu_lock>
| ...
| kvm_free_stage2_pgd()
| mmu->pgt = NULL
| < invalidate MMU >
| ...
| < release kvm->mmu_lock >
[ scheduled ] |
stage2_apply_range() |
< loop to next > |
get, mmu->pgt, check !NULL |
is NULL, return -EINVAL |
__unmap_stage2_range() |
WARN_ON(-EINVAL) <--- entirely spurious - the race was handled
correctly.

Fix the spurious warning by updating stage2_apply_range() to no longer
treat concurrent PGT teardown on lock release as an error - whether the
walker is tearing down page tables or doing something else this is a
legitimate reason to abort the operation without error.

This keeps the warning in place for all other circumstances.

In practice only __unmap_stage2_range() actually does anything with the
error so this only impacts that.
Published: 2026-10-18
Score: n/a
EPSS: n/a
KEV: No
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History

Sun, 18 Oct 2026 15:30:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: Fix spurious warning for benign stage 2 teardown race kvmtool was used to establish an L1 guest with 8 CPUs and 8 GiB of RAM, an L2 guest with 4 CPUs and 4 GiB of RAM and an L3 guest with 2 CPUs and 2 GiB of RAM, all of which was then exited. Under memory pressure in the L0 host warnings were observed due to migration triggered by compaction: WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80, CPU#5: kcompactd0/66 Which was, in turn, triggered by an MMU notifier for the host invalidation: mmu_notifier_invalidate_range_start() -> ... -> kvm_mmu_notifier_invalidate_range_start() -> kvm_mmu_unmap_gfn_range() -> kvm_unmap_gfn_range() -> kvm_nested_s2_unmap() -> kvm_stage2_unmap_range() -> __unmap_stage2_range() -> stage2_apply_range() <- -EINVAL, triggering a WARN_ON() Racing with L0's teardown of stage 2 page tables: exit_mm() -> mmput() -> __mmput() -> exit_mmap() -> mmu_notifier_release() -> ... -> kvm_mmu_notifier_release() -> kvm_flush_shadow_all() -> kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd() -> [ acquire kvm->mmu_lock for write ] -> mmu->pgt = NULL [ among other tasks ] -> [ release kvm->mmu_lock for write ] It turns out there is a benign race resulting in a spurious warning: Thread A - notify: migration | Thread B - notify: release -------------------------------|--------------------------------- < kvm->mmu_lock held > | stage2_apply_range() | get mmu->pgt, check !NULL | ... | kvm_arch_flush_shadow_all() cond_resched_rwlock_write(); | < contend, sleep kvm->mmu_lock > < drop kvm->mmu_lock > | < acquire kvm->mmu_lock> | ... | kvm_free_stage2_pgd() | mmu->pgt = NULL | < invalidate MMU > | ... | < release kvm->mmu_lock > [ scheduled ] | stage2_apply_range() | < loop to next > | get, mmu->pgt, check !NULL | is NULL, return -EINVAL | __unmap_stage2_range() | WARN_ON(-EINVAL) <--- entirely spurious - the race was handled correctly. Fix the spurious warning by updating stage2_apply_range() to no longer treat concurrent PGT teardown on lock release as an error - whether the walker is tearing down page tables or doing something else this is a legitimate reason to abort the operation without error. This keeps the warning in place for all other circumstances. In practice only __unmap_stage2_range() actually does anything with the error so this only impacts that.
Title KVM: arm64: Fix spurious warning for benign stage 2 teardown race
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-10-18T15:15:20.882Z

Reserved: 2026-09-25T10:25:14.351Z

Link: CVE-2026-98468

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