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CVE-2026-53359 ("Januscape") is a use-after-free vulnerability in the Linux kernel's KVM/x86 shadow MMU shadow paging code. The bug is in shadow page reuse logic in kvm_mmu_get_child_sp(): after an earlier fix for unexpected GFN mismatches, KVM could still incorrectly reuse a child shadow page when the guest frame number matched but the shadow page role did not. Specifically, a large 2MB mapping can create a kvm_mmu_page with direct=1, while a later 4KB mapping on the new path requires a page with direct=0; because the role was not compared, the existing page could be reused incorrectly. This leads to rmap bookkeeping inconsistencies: a leaf 4KB SPTE records an rmap entry under the walked GFN, but when the child is later zapped, the parent kvm_mmu_page's direct=1 role causes kvm_mmu_page_get_gfn() to compute the wrong GFN and fail to remove the recorded rmap entry. If the memslot is then dropped, the shadow page is freed while stale rmap entries remain. Later host-side operations that walk that GFN, such as dirty logging or MMU notifier invalidation, can dereference an sptep into freed memory, causing a host-kernel use-after-free. The issue affects KVM x86 shadow MMU code shared by Intel and AMD and reportedly existed since Linux 2.6.36-era code.
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A use-after-free vulnerability in Linux KVM's x86 shadow MMU that can be triggered from a guest VM to corrupt host kernel state, causing host crashes and potentially enabling guest-to-host code execution/root compromise on Intel and AMD systems when nested virtualization is enabled.
A use-after-free vulnerability in the Linux kernel KVM x86 shadow paging code that can lead to host kernel memory corruption and potentially host privilege escalation or guest-to-host escape under the right conditions.
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