CVE-2021-47639

CVE-2021-47639 is a high-severity vulnerability in Linux Linux Kernel with a CVSS 3.x base score of 7.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-416.

Key facts

Description

In the Linux kernel, the following vulnerability has been resolved: KVM: x86/mmu: Zap _all_ roots when unmapping gfn range in TDP MMU Zap both valid and invalid roots when zapping/unmapping a gfn range, as KVM must ensure it holds no references to the freed page after returning from the unmap operation. Most notably, the TDP MMU doesn't zap invalid roots in mmu_notifier callbacks. This leads to use-after-free and other issues if the mmu_notifier runs to completion while an invalid root zapper yields as KVM fails to honor the requirement that there must be _no_ references to the page after the mmu_notifier returns. The bug is most easily reproduced by hacking KVM to cause a collision between set_nx_huge_pages() and kvm_mmu_notifier_release(), but the bug exists between kvm_mmu_notifier_invalidate_range_start() and memslot updates as well. Invalidating a root ensures pages aren't accessible by the guest, and KVM won't read or write page data itself, but KVM will trigger e.g. kvm_set_pfn_dirty() when zapping SPTEs, and thus completing a zap of an invalid root _after_ the mmu_notifier returns is fatal. WARNING: CPU: 24 PID: 1496 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:173 [kvm] RIP: 0010:kvm_is_zone_device_pfn+0x96/0xa0 [kvm] Call Trace: <TASK> kvm_set_pfn_dirty+0xa8/0xe0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] zap_gfn_range+0x1f3/0x310 [kvm] kvm_tdp_mmu_zap_invalidated_roots+0x50/0x90 [kvm] kvm_mmu_zap_all_fast+0x177/0x1a0 [kvm] set_nx_huge_pages+0xb4/0x190 [kvm] param_attr_store+0x70/0x100 module_attr_store+0x19/0x30 kernfs_fop_write_iter+0x119/0x1b0 new_sync_write+0x11c/0x1b0 vfs_write+0x1cc/0x270 ksys_write+0x5f/0xe0 do_syscall_64+0x38/0xc0 entry_SYSCALL_64_after_hwframe+0x44/0xae </TASK>

Frequently asked questions

What is CVE-2021-47639?
In the Linux kernel, the following vulnerability has been resolved: KVM: x86/mmu: Zap _all_ roots when unmapping gfn range in TDP MMU Zap both valid and invalid roots when zapping/unmapping a gfn range, as KVM must ensure it holds no references to the freed page after returning from the unmap operation. Most notably, the TDP MMU doesn't zap invalid roots in mmu_notifier callbacks. This leads to use-after-free and other issues if the mmu_notifier runs to completion while an invalid root zapper yields as KVM fails to honor the requirement that there must be _no_ references to the page after the mmu_notifier returns. The bug is most easily reproduced by hacking KVM to cause a collision between set_nx_huge_pages() and kvm_mmu_notifier_release(), but the bug exists between kvm_mmu_notifier_invalidate_range_start() and memslot updates as well. Invalidating a root ensures pages aren't accessible by the guest, and KVM won't read or write page data itself, but KVM will trigger e.g. kvm_set_pfn_dirty() when zapping SPTEs, and thus completing a zap of an invalid root _after_ the mmu_notifier returns is fatal. WARNING: CPU: 24 PID: 1496 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:173 [kvm] RIP: 0010:kvm_is_zone_device_pfn+0x96/0xa0 [kvm] Call Trace: <TASK> kvm_set_pfn_dirty+0xa8/0xe0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] __handle_changed_spte+0x2ab/0x5e0 [kvm] zap_gfn_range+0x1f3/0x310 [kvm] kvm_tdp_mmu_zap_invalidated_roots+0x50/0x90 [kvm] kvm_mmu_zap_all_fast+0x177/0x1a0 [kvm] set_nx_huge_pages+0xb4/0x190 [kvm] param_attr_store+0x70/0x100 module_attr_store+0x19/0x30 kernfs_fop_write_iter+0x119/0x1b0 new_sync_write+0x11c/0x1b0 vfs_write+0x1cc/0x270 ksys_write+0x5f/0xe0 do_syscall_64+0x38/0xc0 entry_SYSCALL_64_after_hwframe+0x44/0xae </TASK>
How severe is CVE-2021-47639?
CVE-2021-47639 has a CVSS 3.x base score of 7.8, rated high severity. It is exploitable over local access with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-47639 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-47639?
CVE-2021-47639 affects Linux Linux Kernel. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-47639?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2021-47639 have an EU (EUVD) identifier?
Yes. CVE-2021-47639 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-34665.
When was CVE-2021-47639 published?
CVE-2021-47639 was published on 2025-02-26 and last updated on 2026-06-17.

References

Affected products (1)

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