CVE-2025-68171 (GCVE-0-2025-68171)

Vulnerability from cvelistv5 – Published: 2025-12-16 13:42 – Updated: 2026-08-05 12:09
VLAI
Title
x86/fpu: Ensure XFD state on signal delivery
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 ]
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 672365477ae8afca5a1cca98c1deb733235e4525 , < eefbfb722042fc9210d2e0ac2b063fd1abf51895 (git)
Affected: 672365477ae8afca5a1cca98c1deb733235e4525 , < 1811c610653c0cd21cc9add14595b7cffaeca511 (git)
Affected: 672365477ae8afca5a1cca98c1deb733235e4525 , < 5b2619b488f1d08b960c43c6468dd0759e8b3035 (git)
Affected: 672365477ae8afca5a1cca98c1deb733235e4525 , < 3f735419c4b43cde42e6d408db39137b82474e31 (git)
Affected: 672365477ae8afca5a1cca98c1deb733235e4525 , < 388eff894d6bc5f921e9bfff0e4b0ab2684a96e9 (git)
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Linux Linux Affected: 5.16
Unaffected: 0 , < 5.16 (semver)
Unaffected: 6.1.159 , ≤ 6.1.* (semver)
Unaffected: 6.6.117 , ≤ 6.6.* (semver)
Unaffected: 6.12.58 , ≤ 6.12.* (semver)
Unaffected: 6.17.8 , ≤ 6.17.* (semver)
Unaffected: 6.18 , ≤ * (original_commit_for_fix)
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{
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          "product": "Linux",
          "programFiles": [
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              "version": "6.18",
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      "cpeApplicability": [
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          "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 ]"
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            "version": "3.1"
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              "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."
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        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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    "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"
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      "date": "2026-08-21",
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    "redhat_vex": {
      "aggregate_severity": "Low",
      "current_release_date": "2026-07-30T10:07:41+00:00",
      "cve": "CVE-2025-68171",
      "id": "CVE-2025-68171",
      "initial_release_date": "2025-12-16T00:00:00+00:00",
      "product_status:known_affected": "232",
      "product_status:known_not_affected": "42",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: x86/fpu: Ensure XFD state on signal delivery",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-68171.json",
      "version": "3"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.