CVE-2026-53359 in Linux
Summary
by MITRE • 07/04/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: Fix shadow paging use-after-free due to unexpected role
Commit 0cb2af2ea66ad ("KVM: x86: Fix shadow paging use-after-free due to unexpected GFN") fixed a shadow paging mismatch between stored and computed GFNs; the bug could be triggered by changing a PDE mapping from outside the guest, and then deleting a memslot. The rmap_remove() call would miss entries created after the PDE change because the GFN of the leaf SPTE does not match the GFN of the struct kvm_mmu_page.
A similar hole however remains if the modified PDE points to a non-leaf page. In this case the gfn can be made to match, but the role does not match: the original large 2MB page creates a kvm_mmu_page with direct=1, while the new 4KB needs a kvm_mmu_page with direct=0. However, kvm_mmu_get_child_sp() does not compare the role, and therefore reuses the page.
The next step is installing a leaf (4KB) SPTE on the new path which records an rmap entry under the gfn resolved by the walk. But when that child is zapped its parent kvm_mmu_page has direct=1 and kvm_mmu_page_get_gfn() computes the gfn for the 4KB page as sp->gfn + index instead of using sp->shadowed_translation[] (or sp->gfns[]
in older kernels). It therefore fails to remove the recorded entry.
When the memslot is dropped the shadow page is freed but the rmap entry survives, as in the scenario that was already fixed. Code that later walks that gfn (dirty logging, MMU notifier invalidation, and so on) dereferences an sptep that lies in the freed page, causing the use-after-free.
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Analysis
by VulDB Data Team • 07/04/2026
This vulnerability exists within the Linux kernel's KVM virtualization subsystem, specifically affecting x86 architecture implementations where shadow paging mechanisms are employed to manage guest memory translations. The issue stems from improper handling of memory management unit pages during PDE (Page Directory Entry) modifications that occur outside of guest execution contexts. The root cause involves a mismatch between the expected and actual role attributes of memory management unit pages, creating conditions where use-after-free scenarios can manifest when memory slots are removed from the system.
The technical flaw manifests in the kvm_mmu_get_child_sp() function which fails to properly validate role compatibility when reusing existing memory management unit pages. When a PDE transition occurs from a large 2MB page to a 4KB page, the original page structure maintains direct=1 while the new structure requires direct=0, yet the system incorrectly reuses the old page structure without proper role verification. This creates a fundamental inconsistency where the gfn (guest frame number) values may appear to match but the underlying page role attributes remain incompatible.
The operational impact of this vulnerability becomes apparent when subsequent operations attempt to install leaf SPTE (shadow page table entries) on the new mapping path. The system records rmap (reverse mapping) entries under gfn values computed through the walk process, but when these child pages are subsequently zapped, the parent page's direct=1 attribute causes incorrect gfn computation. Specifically, the kvm_mmu_page_get_gfn() function uses sp->gfn + index calculation instead of the proper sp->shadowed_translation[] or sp->gfns[] array access that would correctly account for the 4KB page structure.
When a memslot is eventually dropped from the system, the shadow page memory gets freed but critical rmap entries remain in memory due to the failed removal operation. Subsequent memory management operations such as dirty logging or MMU notifier invalidation attempts to traverse these gfn values and dereference sptep (shadow page table entry pointers) that now point to freed memory locations. This creates a classic use-after-free condition where code accesses memory that has been deallocated, potentially leading to system crashes, data corruption, or privilege escalation opportunities.
The vulnerability aligns with CWE-416 Use After Free, which specifically addresses improper handling of memory after it has been freed, and relates to ATT&CK technique T1059.001 Command and Scripting Interpreter: PowerShell where malicious actors could potentially exploit such memory corruption to execute arbitrary code. The flaw also connects to broader KVM virtualization security concerns involving memory management unit page handling and guest-to-host memory translation integrity.
Mitigation strategies should focus on implementing proper role validation within the kvm_mmu_get_child_sp() function to ensure that page structures with incompatible roles cannot be incorrectly reused during PDE transitions. Additionally, enhanced verification mechanisms must be added to ensure that rmap entry removal operations properly account for all possible page structure variations. The fix requires modifying the shadow paging code to correctly compare role attributes before reusing existing memory management unit pages and ensuring that gfn computations remain consistent regardless of whether direct=0 or direct=1 page structures are involved, preventing the scenario where freed memory locations continue to be referenced through stale rmap entries.
This vulnerability demonstrates a critical gap in virtualization memory management systems where external modifications to PDE mappings can create inconsistent state conditions that persist until memory cleanup operations occur. The complexity of shadow paging mechanisms in KVM makes such issues particularly challenging to detect and prevent, requiring careful attention to role attribute validation during page structure transitions. The fix addresses the specific case where non-leaf page modifications introduce inconsistencies between expected and actual page roles, ensuring that memory management unit pages maintain proper structural integrity throughout their lifecycle.