CVE-2026-23005

// PoC for CVE-2026-23005 - Linux Kernel XSTATE_BV/XFD Sync Issue // This PoC demonstrates the race condition in KVM guest XFD handling #include <stdio.h> #include <stdlib.h> #include <string.h> #include <fcntl.h> #include <errno.h> // Guest-side PoC (run inside VM) void guest_poc() { printf("[*] CVE-2026-23005 PoC - Guest Trigger\n"); printf("[*] Setting XFD[18] = 1 to disable AMX\n"); // Write to MSR_IA32_XFD to disable AMX feature // XFD MSR index: 0x000001d4 unsigned long xfd_msr = 0x000001d4; unsigned long xfd_value = (1UL << 18); // Disable AMX-TILE // This WRMSR will trigger the vulnerability if: // 1. Host IRQ occurs before fpu_update_guest_xfd() // 2. kernel_fpu_begin() is called during IRQ handling printf("[*] Executing WRMSR to set XFD[18]=1\n"); // Simulate the MSR write // __asm__ volatile("wrmsr" :: "c"(xfd_msr), "a"(xfd_value & 0xFFFFFFFF), "d"(xfd_value >> 32)); printf("[!] If host kernel has the bug, this may trigger #NM and panic\n"); } // Host-side simulation void host_simulation() { printf("\n[*] Host-side Race Condition Simulation:\n"); printf("[*] 1. Guest executes WRMSR(MSR_IA32_XFD) with XFD[18]=1\n"); printf("[*] 2. VM-Exit triggered\n"); printf("[*] 3. Host IRQ arrives before fpu_update_guest_xfd()\n"); printf("[*] 4. IRQ handler calls kernel_fpu_begin()\n"); printf("[*] 5. save_fpregs_to_fpstate() saves FPU with guest XFD\n"); printf("[*] 6. XSTATE_BV[18]=1 but XFD[18]=1 (inconsistent)\n"); printf("[*] 7. Later XRSTOR triggers #NM exception\n"); printf("[*] 8. Kernel panic!\n"); } int main() { printf("========================================\n"); printf("CVE-2026-23005 PoC - Linux Kernel x86/fpu\n"); printf("XSTATE_BV/XFD Synchronization Issue\n"); printf("========================================\n\n"); guest_poc(); host_simulation(); printf("\n[*] PoC completed. Check for kernel panic in host dmesg.\n"); return 0; }

{"cve": {"id": "CVE-2026-23005", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-01-25T15:15:55.377", "lastModified": "2026-03-25T19:22:06.410", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/fpu: Clear XSTATE_BV[i] in guest XSAVE state whenever XFD[i]=1\n\nWhen loading guest XSAVE state via KVM_SET_XSAVE, and when updating XFD in\nresponse to a guest WRMSR, clear XFD-disabled features in the saved (or to\nbe restored) XSTATE_BV to ensure KVM doesn't attempt to load state for\nfeatures that are disabled via the guest's XFD. Because the kernel\nexecutes XRSTOR with the guest's XFD, saving XSTATE_BV[i]=1 with XFD[i]=1\nwill cause XRSTOR to #NM and panic the kernel.\n\nE.g. if fpu_update_guest_xfd() sets XFD without clearing XSTATE_BV:\n\n ------------[ cut here ]------------\n WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#29: amx_test/848\n Modules linked in: kvm_intel kvm irqbypass\n 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\n Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n RIP: 0010:exc_device_not_available+0x101/0x110\n Call Trace:\n <TASK>\n asm_exc_device_not_available+0x1a/0x20\n RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90\n switch_fpu_return+0x4a/0xb0\n kvm_arch_vcpu_ioctl_run+0x1245/0x1e40 [kvm]\n kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]\n __x64_sys_ioctl+0x8f/0xd0\n do_syscall_64+0x62/0x940\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n </TASK>\n ---[ end trace 0000000000000000 ]---\n\nThis can happen if the guest executes WRMSR(MSR_IA32_XFD) to set XFD[18] = 1,\nand a host IRQ triggers kernel_fpu_begin() prior to the vmexit handler's\ncall to fpu_update_guest_xfd().\n\nand if userspace stuffs XSTATE_BV[i]=1 via KVM_SET_XSAVE:\n\n ------------[ cut here ]------------\n WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#14: amx_test/867\n Modules linked in: kvm_intel kvm irqbypass\n 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\n Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n RIP: 0010:exc_device_not_available+0x101/0x110\n Call Trace:\n <TASK>\n asm_exc_device_not_available+0x1a/0x20\n RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90\n fpu_swap_kvm_fpstate+0x6b/0x120\n kvm_load_guest_fpu+0x30/0x80 [kvm]\n kvm_arch_vcpu_ioctl_run+0x85/0x1e40 [kvm]\n kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]\n __x64_sys_ioctl+0x8f/0xd0\n do_syscall_64+0x62/0x940\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n </TASK>\n ---[ end trace 0000000000000000 ]---\n\nThe new behavior is consistent with the AMX architecture. Per Intel's SDM,\nXSAVE saves XSTATE_BV as '0' for components that are disabled via XFD\n(and non-compacted XSAVE saves the initial configuration of the state\ncomponent):\n\n If XSAVE, XSAVEC, XSAVEOPT, or XSAVES is saving the state component i,\n the instruction does not generate #NM when XCR0[i] = IA32_XFD[i] = 1;\n instead, it operates as if XINUSE[i] = 0 (and the state component was\n in its initial state): it saves bit i of XSTATE_BV field of the XSAVE\n header as 0; in addition, XSAVE saves the initial configuration of the\n state component (the other instructions do not save state component i).\n\nAlternatively, KVM could always do XRSTOR with XFD=0, e.g. by using\na constant XFD based on the set of enabled features when XSAVEing for\na struct fpu_guest. However, having XSTATE_BV[i]=1 for XFD-disabled\nfeatures can only happen in the above interrupt case, or in similar\nscenarios involving preemption on preemptible kernels, because\nfpu_swap_kvm_fpstate()'s call to save_fpregs_to_fpstate() saves the\noutgoing FPU state with the current XFD; and that is (on all but the\nfirst WRMSR to XFD) the guest XFD.\n\nTherefore, XFD can only go out of sync with XSTATE_BV in the above\ninterrupt case, or in similar scenarios involving preemption on\npreemptible kernels, and it we can consider it (de facto) part of KVM\nABI that KVM_GET_XSAVE returns XSTATE_BV[i]=0 for XFD-disabled features.\n\n[Move clea\n---truncated---"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "NVD-CWE-noinfo"}]}], "configurations": [{"nodes": [{"operator": ... (truncated)