CVE-2026-53345 in Linux
Summary
by MITRE • 07/01/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: Don't WARN if memory is dirtied without a vCPU when the VM is dying
When marking a page dirty, complain about not having a running/loaded vCPU if and only if the VM is still alive, i.e. its refcount is non-zero. This will allow fixing a memory leak for x86 SEV-ES guests without hitting what is effectively a false positive on the WARN.
For some SEV-ES VM-Exits, KVM keeps a writable mapping of a guest page across an exit to userspace, and typically unmaps the page on the next KVM_RUN. But if userspace never calls KVM_RUN after such an exit, then KVM needs to unmap the page when the vCPU is destroyed, which in turn triggers the WARN about not having a running vCPU.
Alternatively, SEV-ES could temporarily load the vCPU to suppress the WARN, as is done in nested_vmx_free_vcpu() (but for completely unrelated reasons; suppressing WARN from nested_put_vmcs12_pages() is pure happenstance). But loading a vCPU during destruction is gross (ideally nVMX code would be cleaned up), risks complicating the SEV-ES code (KVM would need to ensure the temporarily load()+put() only runs when the vCPU isn't already loaded), and is ultimately pointless.
The motivation for the WARN is to guard against KVM dirtying guest memory without pushing the corresponding GFN to the active vCPU's dirty ring, e.g. to ensure userspace doesn't miss a dirty page. But for the VM's refcount to reach zero, there can't be _any_ userspace mappings to the dirty ring, as mapping the dirty ring requires doing mmap() on the vCPU FD. I.e. if userspace had a valid mapping for the dirty ring, then the vCPU file and thus the owning VM would still be alive. And so since userspace can't possibly reach the dirty ring, whether or not KVM technically "misses" a push to the dirty ring is irrelevant.
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Analysis
by VulDB Data Team • 07/01/2026
The vulnerability addressed in this linux kernel patch relates to improper handling of memory dirtying operations within the Kernel-based Virtual Machine (KVM) subsystem when managing SEV-ES (Secure Encrypted Virtualization - Encrypted State) guests. This issue specifically manifests during VM shutdown sequences where KVM attempts to clean up resources associated with virtual CPUs while potentially still marking guest memory pages as dirty. The problem stems from a false positive warning mechanism that incorrectly flags memory operations as problematic when the virtual machine is already in the process of termination, creating unnecessary noise and potentially masking legitimate issues.
The technical flaw exists in how KVM determines when to issue warnings about memory dirtiness operations without proper vCPU context. Previously, the kernel would generate a WARN message whenever guest memory was marked as dirty regardless of whether the VM was still active or had already begun its shutdown process. This design decision was intended to prevent potential data loss scenarios where dirty pages might not be properly tracked in the vCPU's dirty ring, which is used by userspace to monitor which guest pages have been modified. However, this approach failed to account for legitimate cleanup operations that occur during VM destruction when no active vCPU context exists.
The operational impact of this vulnerability becomes particularly significant for x86 SEV-ES guests where memory management during VM exits requires complex handling of writable mappings across user space boundaries. When KVM keeps a writable mapping of guest pages across certain VM exits and those mappings are not properly cleaned up due to userspace not calling KVM_RUN, the system attempts to unmap these pages during vCPU destruction. This cleanup process triggers the erroneous warning mechanism even though the VM is already dying and no active userspace processes can access the dirty ring data structure anymore.
This issue directly relates to CWE-704, which covers improper handling of memory operations in kernel space, and intersects with ATT&CK technique T1566 related to credential dumping through virtualization attacks. The vulnerability demonstrates a classic case where defensive programming measures become counterproductive during system shutdown sequences. The patch resolves this by modifying the warning logic to only trigger when the VM is still alive, as indicated by a non-zero reference count, thereby eliminating false positives during legitimate cleanup operations that occur when the VM's refcount reaches zero.
The mitigation strategy implemented in this fix addresses the root cause rather than working around symptoms. Instead of temporarily loading vCPUs during destruction which would introduce additional complexity and potential race conditions, the solution properly distinguishes between active VM operations and shutdown cleanup scenarios. This approach aligns with secure coding principles that emphasize proper state management and context-aware error handling. The referenced nested_vmx_free_vcpu() function demonstrates an alternative approach that was considered but ultimately rejected due to its complexity and potential for introducing new issues.
The fundamental issue highlights the importance of understanding system lifecycle states when implementing defensive programming patterns. During VM destruction, any attempt to access or manipulate data structures that require active vCPU contexts becomes meaningless since userspace mappings to the dirty ring are no longer possible once the VM's reference count reaches zero. This insight aligns with proper resource management practices where cleanup operations should not generate warnings for operations that cannot actually impact system behavior due to the complete shutdown of dependent components. The fix ensures that KVM's warning mechanisms remain effective for legitimate error conditions while eliminating spurious alerts during normal shutdown procedures, thereby improving both system reliability and maintainability.