Linux Kernel up to 6.12.66/6.18.6/6.19-rc5 fpu_update_guest_xfd state issue

Summaryinfo

A vulnerability has been found in Linux Kernel up to 6.12.66/6.18.6/6.19-rc5 and classified as critical. The affected element is the function fpu_update_guest_xfd. The manipulation leads to state issue. This vulnerability is documented as CVE-2026-23005. There is not any exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability, which was classified as critical, has been found in Linux Kernel up to 6.12.66/6.18.6/6.19-rc5. This issue affects the function fpu_update_guest_xfd. The manipulation with an unknown input leads to a state issue vulnerability. Using CWE to declare the problem leads to CWE-371. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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: ------------[ cut here ]------------ 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: ------------[ cut here ]------------ 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 ---truncated---

The advisory is shared at git.kernel.org. The identification of this vulnerability is CVE-2026-23005 since 01/13/2026. The exploitation is known to be difficult. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 03/27/2026).

The vulnerability scanner Nessus provides a plugin with the ID 296538 (Linux Distros Unpatched Vulnerability : CVE-2026-23005), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.12.67, 6.18.7 or 6.19-rc6 eliminates this vulnerability. Applying the patch f577508cc8a0adb8b4ebe9480bba7683b6149930/eea6f395ca502c4528314c8112da9b5d65f685eb/b45f721775947a84996deb5c661602254ce25ce6 is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (296538), EUVD (EUVD-2026-4627) and CERT Bund (WID-SEC-2026-0215). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Debian Linux
• Amazon Linux 2
• Red Hat Enterprise Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• SUSE openSUSE
• RESF Rocky Linux
• Open Source Linux Kernel

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.12.0
• 6.12.1
• 6.12.2
• 6.12.3
• 6.12.4
• 6.12.5
• 6.12.6
• 6.12.7
• 6.12.8
• 6.12.9
• 6.12.10
• 6.12.11
• 6.12.12
• 6.12.13
• 6.12.14
• 6.12.15
• 6.12.16
• 6.12.17
• 6.12.18
• 6.12.19
• 6.12.20
• 6.12.21
• 6.12.22
• 6.12.23
• 6.12.24
• 6.12.25
• 6.12.26
• 6.12.27
• 6.12.28
• 6.12.29
• 6.12.30
• 6.12.31
• 6.12.32
• 6.12.33
• 6.12.34
• 6.12.35
• 6.12.36
• 6.12.37
• 6.12.38
• 6.12.39
• 6.12.40
• 6.12.41
• 6.12.42
• 6.12.43
• 6.12.44
• 6.12.45
• 6.12.46
• 6.12.47
• 6.12.48
• 6.12.49
• 6.12.50
• 6.12.51
• 6.12.52
• 6.12.53
• 6.12.54
• 6.12.55
• 6.12.56
• 6.12.57
• 6.12.58
• 6.12.59
• 6.12.60
• 6.12.61
• 6.12.62
• 6.12.63
• 6.12.64
• 6.12.65
• 6.12.66
• 6.18.0
• 6.18.1
• 6.18.2
• 6.18.3
• 6.18.4
• 6.18.5
• 6.18.6
• 6.19-rc1
• 6.19-rc2
• 6.19-rc3
• 6.19-rc4
• 6.19-rc5

License

• open-source

Website

• Vendor: https://www.kernel.org/

CPE 2.3info

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Entryinfo

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