FKIE_CVE-2025-37964

Vulnerability from fkie_nvd - Published: 2025-05-20 16:15 - Updated: 2025-12-16 20:30
Severity ?
5.5 (Medium) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: x86/mm: Eliminate window where TLB flushes may be inadvertently skipped tl;dr: There is a window in the mm switching code where the new CR3 is set and the CPU should be getting TLB flushes for the new mm. But should_flush_tlb() has a bug and suppresses the flush. Fix it by widening the window where should_flush_tlb() sends an IPI. Long Version: === History === There were a few things leading up to this. First, updating mm_cpumask() was observed to be too expensive, so it was made lazier. But being lazy caused too many unnecessary IPIs to CPUs due to the now-lazy mm_cpumask(). So code was added to cull mm_cpumask() periodically[2]. But that culling was a bit too aggressive and skipped sending TLB flushes to CPUs that need them. So here we are again. === Problem === The too-aggressive code in should_flush_tlb() strikes in this window: // Turn on IPIs for this CPU/mm combination, but only // if should_flush_tlb() agrees: cpumask_set_cpu(cpu, mm_cpumask(next)); next_tlb_gen = atomic64_read(&next->context.tlb_gen); choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush); load_new_mm_cr3(need_flush); // ^ After 'need_flush' is set to false, IPIs *MUST* // be sent to this CPU and not be ignored. this_cpu_write(cpu_tlbstate.loaded_mm, next); // ^ Not until this point does should_flush_tlb() // become true! should_flush_tlb() will suppress TLB flushes between load_new_mm_cr3() and writing to 'loaded_mm', which is a window where they should not be suppressed. Whoops. === Solution === Thankfully, the fuzzy "just about to write CR3" window is already marked with loaded_mm==LOADED_MM_SWITCHING. Simply checking for that state in should_flush_tlb() is sufficient to ensure that the CPU is targeted with an IPI. This will cause more TLB flush IPIs. But the window is relatively small and I do not expect this to cause any kind of measurable performance impact. Update the comment where LOADED_MM_SWITCHING is written since it grew yet another user. Peter Z also raised a concern that should_flush_tlb() might not observe 'loaded_mm' and 'is_lazy' in the same order that switch_mm_irqs_off() writes them. Add a barrier to ensure that they are observed in the order they are written.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/02ad4ce144bd27f71f583f667fdf3b3ba0753477Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/12f703811af043d32b1c8a30001b2fa04d5cd0acPatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/399ec9ca8fc4999e676ff89a90184ec40031cf59Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/d41072906abec8bb8e01ed16afefbaa558908c89Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/d87392094f96e162fa5fa5a8640d70cc0952806fPatch
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/fea4e317f9e7e1f449ce90dedc27a2d2a95bee5aPatch
af854a3a-2127-422b-91ae-364da2661108https://lists.debian.org/debian-lts-announce/2025/08/msg00010.htmlThird Party Advisory
Impacted products
Vendor Product Version
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel *
linux linux_kernel 6.15
linux linux_kernel 6.15
linux linux_kernel 6.15
linux linux_kernel 6.15
linux linux_kernel 6.15
debian debian_linux 11.0
To clipboard 

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      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Eliminate window where TLB flushes may be inadvertently skipped\n\ntl;dr: There is a window in the mm switching code where the new CR3 is\nset and the CPU should be getting TLB flushes for the new mm.  But\nshould_flush_tlb() has a bug and suppresses the flush.  Fix it by\nwidening the window where should_flush_tlb() sends an IPI.\n\nLong Version:\n\n=== History ===\n\nThere were a few things leading up to this.\n\nFirst, updating mm_cpumask() was observed to be too expensive, so it was\nmade lazier.  But being lazy caused too many unnecessary IPIs to CPUs\ndue to the now-lazy mm_cpumask().  So code was added to cull\nmm_cpumask() periodically[2].  But that culling was a bit too aggressive\nand skipped sending TLB flushes to CPUs that need them.  So here we are\nagain.\n\n=== Problem ===\n\nThe too-aggressive code in should_flush_tlb() strikes in this window:\n\n\t// Turn on IPIs for this CPU/mm combination, but only\n\t// if should_flush_tlb() agrees:\n\tcpumask_set_cpu(cpu, mm_cpumask(next));\n\n\tnext_tlb_gen = atomic64_read(\u0026next-\u003econtext.tlb_gen);\n\tchoose_new_asid(next, next_tlb_gen, \u0026new_asid, \u0026need_flush);\n\tload_new_mm_cr3(need_flush);\n\t// ^ After \u0027need_flush\u0027 is set to false, IPIs *MUST*\n\t// be sent to this CPU and not be ignored.\n\n        this_cpu_write(cpu_tlbstate.loaded_mm, next);\n\t// ^ Not until this point does should_flush_tlb()\n\t// become true!\n\nshould_flush_tlb() will suppress TLB flushes between load_new_mm_cr3()\nand writing to \u0027loaded_mm\u0027, which is a window where they should not be\nsuppressed.  Whoops.\n\n=== Solution ===\n\nThankfully, the fuzzy \"just about to write CR3\" window is already marked\nwith loaded_mm==LOADED_MM_SWITCHING.  Simply checking for that state in\nshould_flush_tlb() is sufficient to ensure that the CPU is targeted with\nan IPI.\n\nThis will cause more TLB flush IPIs.  But the window is relatively small\nand I do not expect this to cause any kind of measurable performance\nimpact.\n\nUpdate the comment where LOADED_MM_SWITCHING is written since it grew\nyet another user.\n\nPeter Z also raised a concern that should_flush_tlb() might not observe\n\u0027loaded_mm\u0027 and \u0027is_lazy\u0027 in the same order that switch_mm_irqs_off()\nwrites them.  Add a barrier to ensure that they are observed in the\norder they are written."
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      "lang": "es",
      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: x86/mm: Eliminar la ventana donde los vaciados de TLB pueden omitirse inadvertidamente tl;dr: Hay una ventana en el c\u00f3digo de conmutaci\u00f3n mm donde se establece el nuevo CR3 y la CPU deber\u00eda obtener vaciados de TLB para el nuevo mm. Pero should_flush_tlb() tiene un error y suprime el vaciado. Arr\u00e9glelo ampliando la ventana donde should_flush_tlb() env\u00eda una IPI. Versi\u00f3n larga: === Historial === Hubo algunas cosas que llevaron a esto. Primero, se observ\u00f3 que actualizar mm_cpumask() era demasiado costoso, por lo que se hizo m\u00e1s perezoso. Pero ser perezoso caus\u00f3 demasiados IPI innecesarios a las CPU debido al ahora perezoso mm_cpumask(). Entonces se agreg\u00f3 c\u00f3digo para descartar mm_cpumask() peri\u00f3dicamente[2]. Pero ese descarte fue demasiado agresivo y omiti\u00f3 el env\u00edo de vaciados de TLB a las CPU que los necesitan. As\u00ed que aqu\u00ed estamos de nuevo. === Problema === El c\u00f3digo demasiado agresivo en should_flush_tlb() ataca en esta ventana: // Activa las IPI para esta combinaci\u00f3n de CPU/mm, pero solo si should_flush_tlb() est\u00e1 de acuerdo: cpumask_set_cpu(cpu, mm_cpumask(next)); next_tlb_gen = atomic64_read(\u0026amp;next-\u0026gt;context.tlb_gen); choose_new_asid(next, next_tlb_gen, \u0026amp;new_asid, \u0026amp;need_flush); load_new_mm_cr3(need_flush); // ^ Despu\u00e9s de que \u0027need_flush\u0027 se establece en falso, las IPI *DEBEN* // enviarse a esta CPU y no ignorarse. this_cpu_write(cpu_tlbstate.loaded_mm, next); // ^ \u00a1No es hasta este punto que should_flush_tlb() // se vuelve verdadero! should_flush_tlb() suprimir\u00e1 los vaciados de TLB entre load_new_mm_cr3() y la escritura en \u0027loaded_mm\u0027, que es una ventana donde no deber\u00edan suprimirse. \u00a1Uy! === Soluci\u00f3n === Afortunadamente, la ventana difusa \"a punto de escribir CR3\" ya est\u00e1 marcada con load_mm==LOADED_MM_SWITCHING. Simplemente comprobar ese estado en should_flush_tlb() es suficiente para asegurar que la CPU est\u00e9 dirigida a un IPI. Esto provocar\u00e1 m\u00e1s IPI de vaciado de TLB. Sin embargo, la ventana es relativamente peque\u00f1a y no preveo que esto tenga ning\u00fan impacto medible en el rendimiento. Actualice el comentario donde se escribe LOADED_MM_SWITCHING, ya que ha generado otro usuario. Peter Z tambi\u00e9n plante\u00f3 la preocupaci\u00f3n de que should_flush_tlb() podr\u00eda no observar \u0027loaded_mm\u0027 e \u0027is_lazy\u0027 en el mismo orden en que switch_mm_irqs_off() los escribe. A\u00f1ade una barrera para garantizar que se observen en el orden en que est\u00e1n escritos."
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  "id": "CVE-2025-37964",
  "lastModified": "2025-12-16T20:30:11.323",
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          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 5.5,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
          "version": "3.1"
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        "exploitabilityScore": 1.8,
        "impactScore": 3.6,
        "source": "nvd@nist.gov",
        "type": "Primary"
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  "published": "2025-05-20T16:15:34.683",
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  "vulnStatus": "Analyzed",
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  • 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.
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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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