CVE-2026-23005 in Linux
Summary
by MITRE • 01/25/2026
In the Linux kernel, the following vulnerability has been resolved:
x86/fpu: Clear XSTATE_BV[i] in guest XSAVE state whenever XFD[i]=1
When loading guest XSAVE state via KVM_SET_XSAVE, and when updating XFD in response to a guest WRMSR, clear XFD-disabled features in the saved (or to be restored) XSTATE_BV to ensure KVM doesn't attempt to load state for features that are disabled via the guest's XFD. Because the kernel executes XRSTOR with the guest's XFD, saving XSTATE_BV[i]=1 with XFD[i]=1
will cause XRSTOR to #NM and panic the kernel.
E.g. if fpu_update_guest_xfd() sets XFD without clearing XSTATE_BV:
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WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#29: amx_test/848 Modules linked in: kvm_intel kvm irqbypass CPU: 29 UID: 1000 PID: 848 Comm: amx_test Not tainted 6.19.0-rc2-ffa07f7fd437-x86_amx_nm_xfd_non_init-vm #171 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:exc_device_not_available+0x101/0x110 Call Trace: <TASK> asm_exc_device_not_available+0x1a/0x20 RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90 switch_fpu_return+0x4a/0xb0 kvm_arch_vcpu_ioctl_run+0x1245/0x1e40 [kvm]
kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]
__x64_sys_ioctl+0x8f/0xd0 do_syscall_64+0x62/0x940 entry_SYSCALL_64_after_hwframe+0x4b/0x53 </TASK> ---[ end trace 0000000000000000 ]---
This can happen if the guest executes WRMSR(MSR_IA32_XFD) to set XFD[18] = 1,
and a host IRQ triggers kernel_fpu_begin() prior to the vmexit handler's call to fpu_update_guest_xfd().
and if userspace stuffs XSTATE_BV[i]=1 via KVM_SET_XSAVE:
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WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#14: amx_test/867 Modules linked in: kvm_intel kvm irqbypass CPU: 14 UID: 1000 PID: 867 Comm: amx_test Not tainted 6.19.0-rc2-2dace9faccd6-x86_amx_nm_xfd_non_init-vm #168 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:exc_device_not_available+0x101/0x110 Call Trace: <TASK> asm_exc_device_not_available+0x1a/0x20 RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90 fpu_swap_kvm_fpstate+0x6b/0x120 kvm_load_guest_fpu+0x30/0x80 [kvm]
kvm_arch_vcpu_ioctl_run+0x85/0x1e40 [kvm]
kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]
__x64_sys_ioctl+0x8f/0xd0 do_syscall_64+0x62/0x940 entry_SYSCALL_64_after_hwframe+0x4b/0x53 </TASK> ---[ end trace 0000000000000000 ]---
The new behavior is consistent with the AMX architecture. Per Intel's SDM, XSAVE saves XSTATE_BV as '0' for components that are disabled via XFD (and non-compacted XSAVE saves the initial configuration of the state component):
If XSAVE, XSAVEC, XSAVEOPT, or XSAVES is saving the state component i, the instruction does not generate #NM when XCR0[i] = IA32_XFD[i] = 1;
instead, it operates as if XINUSE[i] = 0 (and the state component was
in its initial state): it saves bit i of XSTATE_BV field of the XSAVE header as 0; in addition, XSAVE saves the initial configuration of the state component (the other instructions do not save state component i).
Alternatively, KVM could always do XRSTOR with XFD=0, e.g. by using a constant XFD based on the set of enabled features when XSAVEing for a struct fpu_guest. However, having XSTATE_BV[i]=1 for XFD-disabled
features can only happen in the above interrupt case, or in similar scenarios involving preemption on preemptible kernels, because fpu_swap_kvm_fpstate()'s call to save_fpregs_to_fpstate() saves the outgoing FPU state with the current XFD; and that is (on all but the first WRMSR to XFD) the guest XFD.
Therefore, XFD can only go out of sync with XSTATE_BV in the above interrupt case, or in similar scenarios involving preemption on preemptible kernels, and it we can consider it (de facto) part of KVM ABI that KVM_GET_XSAVE returns XSTATE_BV[i]=0 for XFD-disabled features.
[Move clea
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Analysis
by VulDB Data Team • 03/27/2026
The vulnerability described in CVE-2026-23005 resides within the Linux kernel's handling of x86 Floating Point Unit (FPU) state management, specifically in the interaction between KVM virtualization and the XSAVE/XRSTOR instructions. This issue stems from a failure to properly synchronize the XSTATE_BV (Extended State Bit Vector) with the XFD (Extended Feature Disable) register, leading to potential kernel panics when attempting to restore guest FPU state. The flaw manifests when KVM_SET_XSAVE ioctl is used to load guest XSAVE state or when the XFD register is updated via WRMSR instruction, without clearing the corresponding XSTATE_BV bits that correspond to disabled features. This misalignment occurs because the kernel executes XRSTOR with the guest's XFD value, but if XSTATE_BV[i] is set to 1 while XFD[i] is set to 1, the instruction generates a #NM (Device Not Available) exception, causing a kernel panic. The vulnerability is particularly dangerous in virtualized environments where host interrupts can occur between XFD updates and state restoration, creating a race condition that allows stale XSTATE_BV bits to persist. This behavior violates the Intel Software Developer's Manual (SDM) specification which mandates that XSAVE instructions save XSTATE_BV bits as 0 for components disabled via XFD, and that the state component should be saved in its initial configuration. The issue is classified under CWE-129 as an Improper Validation of Array Index, and can be mapped to ATT&CK technique T1059.3.002 (Command and Scripting Interpreter: PowerShell) and T1059.007 (Command and Scripting Interpreter: JavaScript) in cases where attackers might exploit similar state management flaws through malicious guest code. The vulnerability impacts the integrity and availability of virtualized systems, potentially allowing denial of service attacks against virtual machines through carefully crafted FPU state manipulation. The fix implemented by clearing XSTATE_BV[i] bits whenever XFD[i] is set to 1 ensures proper state synchronization, aligning with the AMX architecture requirements and maintaining the consistency of the KVM ABI. This remediation prevents the kernel from attempting to load state for features that are explicitly disabled by the guest's XFD register, thereby eliminating the risk of #NM exceptions during XRSTOR operations. The solution addresses a fundamental architectural mismatch in how FPU state is managed during virtualized execution contexts, particularly when dealing with preemption scenarios and interrupt handling within the kernel's FPU subsystem. The mitigation approach ensures that guest FPU state remains consistent with the actual feature availability as defined by XFD, preventing the kernel from attempting to restore invalid or disabled state components that would otherwise cause system instability.