CVE-2026-90040 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: SEV: Forcefully invalidate SNP VMSA if its backing gmem page is zapped
Wire up a gmem_invalidate_range() call for SNP VMs, and use it to force vCPUs to reload/recheck their guest-provided VMSA if the backing gmem page is being invalidated, e.g. is being PUNCH_HOLE'd. Use the same core logic to handle invalidations as VMX does for the APIC-access page, as the two concepts are nearly identical: shove the physical address of a page into the vCPU's control structure:
1. Snapshot the invalidation sequence counter 2. Grab the pfn (from guest_memfd in this case) 3. Acquire mmu_lock for read 4. Re-request reload if retry is needed, otherwise commit the change.
Note, the re-request action in #4 is necessary as KVM's retry logic is fuzzy, i.e. can get false positives. If the guest_memfd page has been dropped, at some point a subsequent reload will fail to get a PFN from guest_memfd, and KVM will fail KVM_RUN. If the retry was due to a false positive, KVM will retry until there are no relevant MMU notifier events (and will retry in the "outer" loop, i.e. will drop locks and resched as needed).
Note #2! Take care to invalidate the VMSA when a relevant memslot is DELETED or MOVED, as invalidations in response to PUNCH_HOLE are predicated on memslot bindings (KVM doesn't know what GFN range(s) to invalidate without a binding). And more importantly, the VMSA mapping requires a memslot, i.e. must be invalidated if its memslots disappears, regardless of the state of the underlying guest_memfd inode.
Failure to invalidate the vCPU's control.vmsa_pa (which is checked by pre_sev_run()) can prevent KVM from properly freeing the page as firmware will reject the RMPUPDATE to reclaim the page with FAIL_INUSE if the vCPU is actively running, i.e. if VMSA page is in-use. That in turn leads to an RMP #PF on the next use, as the page will still be assigned to the SNP VM.
SEV-SNP: RMPUPDATE failed for PFN 78d198, pg_level: 1, ret: 3 SEV-SNP: PFN 0x78d198, RMP entry: [0xfff0000000144001 - 0x000000000000000f]
CPU: 3 UID: 0 PID: 31345 Comm: sev_snp_vmsa_pu Tainted: G U O Tainted: [U]=USER, [O]=OOT_MODULE
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.86.0-102 01/25/2026 Call Trace: <TASK> dump_stack_lvl+0x54/0x70 rmpupdate+0x12c/0x140 rmp_make_shared+0x3b/0x60 sev_gmem_invalidate+0xe0/0x170 [kvm_amd]
delete_from_page_cache_batch+0x1d8/0x220 truncate_inode_pages_range+0x120/0x3d0 kvm_gmem_fallocate+0x19a/0x270 [kvm]
vfs_fallocate+0x1bc/0x1f0 __x64_sys_fallocate+0x48/0x70 do_syscall_64+0x10a/0x480 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x496c7e </TASK> ------------[ cut here ]------------
SEV: Failed to update RMP entry for PFN 0x78d198 error -14 WARNING: arch/x86/kvm/svm/sev.c:5160 at sev_gmem_invalidate+0x126/0x170 [kvm_amd], CPU#3: sev_snp_vmsa_pu/31345
CPU: 3 UID: 0 PID: 31345 Comm: sev_snp_vmsa_pu Tainted: G U O Tainted: [U]=USER, [O]=OOT_MODULE
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.86.0-102 01/25/2026 RIP: 0010:sev_gmem_invalidate+0x12b/0x170 [kvm_amd]
Call Trace: <TASK> delete_from_page_cache_batch+0x1d8/0x220 truncate_inode_pages_range+0x120/0x3d0 kvm_gmem_fallocate+0x19a/0x270 [kvm]
vfs_fallocate+0x1bc/0x1f0 __x64_sys_fallocate+0x48/0x70 do_syscall_64+0x10a/0x480 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x496c7e </TASK> irq event stamp: 20689 hardirqs last enabled at (20699): [<ffffffff8e76092c>] __console_unlock+0x5c/0x60
hardirqs last disabled at (20708): [<ffffffff8e760911>] __console_unlock+0x41/0x60
softirqs last enabled at (20722): [<ffffffff8e6cd74e>] __irq_exit_rcu+0x7e/0x140
softirqs last disabled at (20717): [<ffffffff8e6cd74e>] __irq_exit_rcu+0x7e/0x140
---[ end trace 0000000000000000 ]---
BUG: unable to handle page fault for address: ffff99 ---truncated---
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel's KVM subsystem, specifically within the AMD Secure Encrypted Virtualization with Secure Nested Paging (SEV-SNP) implementation, contained a critical synchronization flaw regarding the lifecycle management of Guest Memory File descriptors and their associated virtual machine control structures. The vulnerability centers on the handling of the Virtual Machine Save Area (VMSA), which is a guest-provided data structure containing processor state information essential for vCPU execution. When an application utilizing guest_memfd performs operations such as punching holes in memory or deleting/moving memory slots, the underlying physical pages backing these structures may be invalidated or unmapped from the host's perspective. However, prior to this fix, KVM did not forcefully invalidate the VMSA references held by active vCPUs when their backing gmem pages were zapped. This lack of synchronization created a race condition where the hypervisor could attempt to reclaim or modify memory resources that were still actively referenced and potentially in use by running virtual machines.
The technical root cause lies in the failure to wire up the gmem_invalidate_range() call for SNP VMs during page invalidation events. In SEV-SNP environments, the firmware maintains a Reverse Map (RMP) table to track ownership and permissions of physical pages. If a vCPU continues to hold a reference to a VMSA whose backing page has been invalidated by the host but not yet cleared from the vCPU's control structure, subsequent attempts by KVM to free that memory or update its RMP entry will fail. Specifically, the AMD firmware rejects such operations with an error indicating the page is in use (FAIL_INUSE), because it detects that a processor context still references the physical frame number. This discrepancy leaves the system in an inconsistent state where the host believes the resource is available for reuse while the guest hardware context retains active mappings to it.
The operational impact of this vulnerability manifests as severe stability issues and potential denial-of-service conditions within the virtualized environment. When KVM attempts to perform memory management operations like punching holes or deleting memslots, the mismatch between the hypervisor's view and the firmware's RMP state triggers hardware exceptions. As evidenced by kernel logs, these errors often result in RMP update failures followed by page faults (RMP #PF) on subsequent vCPU runs. The system may experience crashes, indicated by BUG: unable to handle page fault messages, or enter unstable states where virtual machines fail to execute correctly due to corrupted control structures. This affects the reliability of cloud infrastructure and enterprise deployments relying on SEV-SNP for confidential computing, as attackers could potentially trigger these conditions through malicious memory allocation patterns to disrupt service availability.
From a security standards perspective, this flaw aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'). The vulnerability arises because multiple execution contexts—the hypervisor's memory management threads and the vCPU hardware context—access shared resources (the VMSA page mappings) without adequate coordination. Furthermore, it relates to CWE-401: Missing Release of Memory after Effective Lifetime if interpreted as a resource leak leading to state corruption, though the primary issue is synchronization rather than simple leakage. In terms of MITRE ATT&CK, this could be leveraged in techniques related to Resource Hijacking or Denial of Service (T1499), where an adversary manipulates memory states to cause system instability or crash virtualized workloads.
To mitigate this vulnerability, the Linux kernel has implemented a robust invalidation mechanism that mirrors logic used for APIC-access pages on VMX platforms. The fix introduces a sequence counter-based approach to ensure safe updates of physical addresses in vCPU control structures. When an invalidation is required, such as during PUNCH_HOLE operations or memslot deletion/movement, the system snapshots the current invalidation sequence counter and retrieves the page frame number from the guest_memfd. It then acquires the MMU lock for reading to check if a reload of the VMSA mapping is necessary. If a retry is needed due to concurrent modifications, the logic handles this gracefully by dropping locks and rescheduling, preventing false positives that could lead to incorrect state updates. Crucially, the fix ensures that invalidation occurs not only on page zapping but also when relevant memslots are deleted or moved, ensuring that VMSA mappings are cleared regardless of the underlying inode's state. Administrators must apply kernel patches containing this KVM SEV-SNP synchronization fix and ensure their hypervisor environments are updated to prevent race conditions during dynamic memory management operations.