CVE-2026-63807
Status: AnalyzedHigh (8.8)—
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86/mmu: Ensure hugepage is in by slot before checking max mapping level
When recovering hugepages in the shadow MMU, verify that the base gfn of the shadow page is actually contained within the target memslot, *before* querying the max mapping level given the shadow page's gfn. Failure to pre-check the validity of the gfn can lead to an out-of-bounds access to the slot's lpage_info (which typically manifests as a host #PF because the lpage_info is vmalloc'd) if the guest creates a hugepage mapping (in its PTEs) that extends "below" the bounds of a memslot.
Read full descriptionShow less
When faulting in memory for a guest, and the size of the guest mapping is greater than KVM's (current) max mapping, then KVM will create a "direct" shadow page (direct in that there are no gPTEs to shadow, and so the target gfn is a direct calculation given the base gfn of the shadow page). The hugepage recovery flow looks for such direct shadow pages, as forcing 4KiB mappings when dirty logging generates the guest > host mapping size case. When the 4KiB restriction is lifted, then KVM can replace the shadow page with a hugepage.
But if KVM originally used a smaller mapping than the guest because the range of memory covered by the guest hugepage exceeds the bounds of a memslot, then KVM will link a direct shadow page with a gfn that is outside the bounds of the memslot being used to fault in memory. The rmap entry added for the leaf mapping is correct and within bounds, but the gfn of the leaf SPTE's parent shadow page will be out of bounds.
Don't bother pre-checking the bounds of the potential hugepage, i.e. don't check that e.g. sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level + 1) is also within the memslot, as the checks performed by kvm_mmu_max_mapping_level() are a superset of the basic bounds checks. I.e. pre-checking the full range would be a dubious micro-optimization.
Technical details traces, logs and code from the original report
BUG: unable to handle page fault for address: ffffc90000806ffc #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 100000067 P4D 100000067 PUD 1002a7067 PMD 10612f067 PTE 0 Oops: Oops: 0000 [#1] SMP CPU: 13 UID: 1000 PID: 757 Comm: mmu_stress_test Not tainted 7.1.0-rc1-48ce1e26eace-x86_pir_to_irr_comments-vm #341 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:kvm_mmu_max_mapping_level+0x79/0x2b0 [kvm] Call Trace: <TASK> kvm_mmu_recover_huge_pages+0x21b/0x320 [kvm] kvm_set_memslot+0x1ee/0x590 [kvm] kvm_set_memory_region.part.0+0x3a1/0x4d0 [kvm] kvm_vm_ioctl+0x9bf/0x15d0 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0xb7/0xbb0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f21c0f1a9bf </TASK>
CVSS
- Version: 3.1
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- Base score: 8.8
Exploitation probability (EPSS)
- Probability of exploitation in the next 30 days: 0.14%
- Percentile among all scored CVEs: 3
- Score date: 10/5/2026
EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).
🎯 ATT&CK techniques
How this vulnerability is exploited and what the attacker gains, in MITRE ATT&CK terms.
- Exploitation
T1068Exploitation for Privilege Escalationprivilege escalation85 % - Primary impact
T1059Command and Scripting Interpreterexecution70 %
Vulnerabilidad local (AV:L/PR:L) sin interacción: escalada de privilegios al permitir acceso fuera de bounds en memoria del kernel. Código remoto posible mediante escritura de memoria de supervisor.
Inferred by our analysis agent from the official description, CVSS vector and CWE, and checked by a supervisor. May contain errors.
🛡️ ATT&CK mitigations that cover these techniques
Affected technologies (1)
CWEs
- CWE-125
References
- https://git.kernel.org/stable/c/18587f9831612e24cd8f24be1ec15478feff7abc
- https://git.kernel.org/stable/c/48b91ed7e22bb82571c34f8b80b6ecdc90a6fab8
- https://git.kernel.org/stable/c/5cab1c989f938f5e1b9a0de66486f1fc2c28479b
- https://git.kernel.org/stable/c/7b52008023b7facf40fba3ebe92449bda8ea53b9
- https://git.kernel.org/stable/c/b2ae3245ea44dccaa9af676b6747476951883318
- https://git.kernel.org/stable/c/c5c29b3c268332afe67d598a034c58344540ed92
- https://git.kernel.org/stable/c/ef057cbf825e03b63f6edf5980f96abf3c53089d
Raw JSON (NVD)
Show
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"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86/mmu: Ensure hugepage is in by slot before checking max mapping level\n\nWhen recovering hugepages in the shadow MMU, verify that the base gfn of\nthe shadow page is actually contained within the target memslot, *before*\nquerying the max mapping level given the shadow page's gfn. Failure to\npre-check the validity of the gfn can lead to an out-of-bounds access to\nthe slot's lpage_info (which typically manifests as a host #PF because the\nlpage_info is vmalloc'd) if the guest creates a hugepage mapping (in its\nPTEs) that extends \"below\" the bounds of a memslot.\n\nWhen faulting in memory for a guest, and the size of the guest mapping is\ngreater than KVM's (current) max mapping, then KVM will create a \"direct\"\nshadow page (direct in that there are no gPTEs to shadow, and so the target\ngfn is a direct calculation given the base gfn of the shadow page). The\nhugepage recovery flow looks for such direct shadow pages, as forcing 4KiB\nmappings when dirty logging generates the guest > host mapping size case.\nWhen the 4KiB restriction is lifted, then KVM can replace the shadow page\nwith a hugepage.\n\nBut if KVM originally used a smaller mapping than the guest because the\nrange of memory covered by the guest hugepage exceeds the bounds of a\nmemslot, then KVM will link a direct shadow page with a gfn that is outside\nthe bounds of the memslot being used to fault in memory. The rmap entry\nadded for the leaf mapping is correct and within bounds, but the gfn of the\nleaf SPTE's parent shadow page will be out of bounds.\n\n BUG: unable to handle page fault for address: ffffc90000806ffc\n #PF: supervisor read access in kernel mode\n #PF: error_code(0x0000) - not-present page\n PGD 100000067 P4D 100000067 PUD 1002a7067 PMD 10612f067 PTE 0\n Oops: Oops: 0000 [#1] SMP\n CPU: 13 UID: 1000 PID: 757 Comm: mmu_stress_test Not tainted 7.1.0-rc1-48ce1e26eace-x86_pir_to_irr_comments-vm #341 PREEMPT\n Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n RIP: 0010:kvm_mmu_max_mapping_level+0x79/0x2b0 [kvm]\n Call Trace:\n <TASK>\n kvm_mmu_recover_huge_pages+0x21b/0x320 [kvm]\n kvm_set_memslot+0x1ee/0x590 [kvm]\n kvm_set_memory_region.part.0+0x3a1/0x4d0 [kvm]\n kvm_vm_ioctl+0x9bf/0x15d0 [kvm]\n __x64_sys_ioctl+0x8a/0xd0\n do_syscall_64+0xb7/0xbb0\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n RIP: 0033:0x7f21c0f1a9bf\n </TASK>\n\nDon't bother pre-checking the bounds of the potential hugepage, i.e. don't\ncheck that e.g. sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level + 1) is also\nwithin the memslot, as the checks performed by kvm_mmu_max_mapping_level()\nare a superset of the basic bounds checks. I.e. pre-checking the full\nrange would be a dubious micro-optimization."
}
],
"lastModified": "2026-08-17T05:17:17.583",
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