CVE-2025-68171 (GCVE-0-2025-68171)
Vulnerability from cvelistv5
Published
2025-12-16 13:42
Modified
2026-08-05 12:09
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: x86/fpu: Ensure XFD state on signal delivery Sean reported [1] the following splat when running KVM tests: WARNING: CPU: 232 PID: 15391 at xfd_validate_state+0x65/0x70 Call Trace: <TASK> fpu__clear_user_states+0x9c/0x100 arch_do_signal_or_restart+0x142/0x210 exit_to_user_mode_loop+0x55/0x100 do_syscall_64+0x205/0x2c0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 Chao further identified [2] a reproducible scenario involving signal delivery: a non-AMX task is preempted by an AMX-enabled task which modifies the XFD MSR. When the non-AMX task resumes and reloads XSTATE with init values, a warning is triggered due to a mismatch between fpstate::xfd and the CPU's current XFD state. fpu__clear_user_states() does not currently re-synchronize the XFD state after such preemption. Invoke xfd_update_state() which detects and corrects the mismatch if there is a dynamic feature. This also benefits the sigreturn path, as fpu__restore_sig() may call fpu__clear_user_states() when the sigframe is inaccessible. [ dhansen: minor changelog munging ]
References
Impacted products
Vendor Product Version
Linux Linux Version: 672365477ae8afca5a1cca98c1deb733235e4525
Version: 672365477ae8afca5a1cca98c1deb733235e4525
Version: 672365477ae8afca5a1cca98c1deb733235e4525
Version: 672365477ae8afca5a1cca98c1deb733235e4525
Version: 672365477ae8afca5a1cca98c1deb733235e4525
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{
  "containers": {
    "cna": {
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          "product": "Linux",
          "programFiles": [
            "arch/x86/kernel/fpu/core.c"
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          "vendor": "Linux",
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              "version": "6.17.8",
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              "version": "6.18",
              "versionType": "original_commit_for_fix"
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      ],
      "cpeApplicability": [
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          "nodes": [
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/fpu: Ensure XFD state on signal delivery\n\nSean reported [1] the following splat when running KVM tests:\n\n   WARNING: CPU: 232 PID: 15391 at xfd_validate_state+0x65/0x70\n   Call Trace:\n    \u003cTASK\u003e\n    fpu__clear_user_states+0x9c/0x100\n    arch_do_signal_or_restart+0x142/0x210\n    exit_to_user_mode_loop+0x55/0x100\n    do_syscall_64+0x205/0x2c0\n    entry_SYSCALL_64_after_hwframe+0x4b/0x53\n\nChao further identified [2] a reproducible scenario involving signal\ndelivery: a non-AMX task is preempted by an AMX-enabled task which\nmodifies the XFD MSR.\n\nWhen the non-AMX task resumes and reloads XSTATE with init values,\na warning is triggered due to a mismatch between fpstate::xfd and the\nCPU\u0027s current XFD state. fpu__clear_user_states() does not currently\nre-synchronize the XFD state after such preemption.\n\nInvoke xfd_update_state() which detects and corrects the mismatch if\nthere is a dynamic feature.\n\nThis also benefits the sigreturn path, as fpu__restore_sig() may call\nfpu__clear_user_states() when the sigframe is inaccessible.\n\n[ dhansen: minor changelog munging ]"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The vulnerable code is reached only through local task-context paths \u2014 signal delivery from `arch_do_signal_or_restart()` and the `rt_sigreturn` syscall \u2014 requiring the attacker to execute code on the machine. No network or adjacent-network reachability exists.\nAC:L - The attacker controls both sides of the race: it runs its own AMX-enabled and non-AMX threads, pins them to a chosen CPU, floods signals, and can force a scheduling point in the exact window between `copy_fpstate_to_sigframe()` and `fpu__clear_user_states()` by faulting on userfaultfd-backed sigframe pages. AMX/XFD-capable Xeons (Sapphire Rapids and later) are mainstream cloud/datacenter hardware, so the required configuration is a normal deployment rather than a rare precondition.\nPR:L - Any unprivileged local user can deliver signals to its own threads, call `arch_prctl(ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA)`, set CPU affinity, and invoke `rt_sigreturn` with an inaccessible sigframe \u2014 there is no capability, namespace, or LSM check anywhere on the path.\nUI:N - The attacker triggers the entire sequence from its own processes via signals and scheduling; no action by any other user is required.\nS:U - The XFD desync and its consequences stay within the kernel\u0027s own security authority on the host \u2014 there is no guest-to-host escape, IOMMU bypass, or other crossing of a security-authority boundary.\nC:H - The stale XFD causes the XRSTOR that is specifically responsible for scrubbing dynamic-feature state left by the previous task to skip XTILEDATA, and `fpregs_mark_activate()` suppresses the later corrective restore, so up to 8 KiB of another process\u0027s AMX tile register content survives into the attacker\u0027s context and can be dumped with a userspace `XSAVE` after re-enabling XFD via #NM; the debug WARN additionally discloses kernel addresses.\nI:H - The same primitive works in reverse \u2014 attacker-chosen tile register content is left un-scrubbed and becomes the operating state of a victim AMX process, silently corrupting its computation \u2014 and the desync also lets a task use AMX with no `ARCH_REQ_XCOMP_PERM` grant while its fpstate has no XTILEDATA storage, bypassing the dynamic-xstate permission model and silently discarding FPU state across context switches.\nA:H - The mismatch trips `WARN_ON_ONCE()` in `xfd_validate_state()` from both `os_xrstor()` and the subsequent `os_xsave()` (the reported splat), which is a fatal panic on systems booted with `panic_on_warn=1`, and leaves the FPU state machine persistently inconsistent with the hardware."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:09:59.980Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/eefbfb722042fc9210d2e0ac2b063fd1abf51895"
        },
        {
          "url": "https://git.kernel.org/stable/c/1811c610653c0cd21cc9add14595b7cffaeca511"
        },
        {
          "url": "https://git.kernel.org/stable/c/5b2619b488f1d08b960c43c6468dd0759e8b3035"
        },
        {
          "url": "https://git.kernel.org/stable/c/3f735419c4b43cde42e6d408db39137b82474e31"
        },
        {
          "url": "https://git.kernel.org/stable/c/388eff894d6bc5f921e9bfff0e4b0ab2684a96e9"
        }
      ],
      "title": "x86/fpu: Ensure XFD state on signal delivery",
      "x_generator": {
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      }
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  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2025-68171",
    "datePublished": "2025-12-16T13:42:51.121Z",
    "dateReserved": "2025-12-16T13:41:40.251Z",
    "dateUpdated": "2026-08-05T12:09:59.980Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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