CVE-2025-39881 (GCVE-0-2025-39881)

Vulnerability from cvelistv5 – Published: 2025-09-23 06:00 – Updated: 2026-08-05 12:05
VLAI
Title
kernfs: Fix UAF in polling when open file is released
Summary
In the Linux kernel, the following vulnerability has been resolved: kernfs: Fix UAF in polling when open file is released A use-after-free (UAF) vulnerability was identified in the PSI (Pressure Stall Information) monitoring mechanism: BUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140 Read of size 8 at addr ffff3de3d50bd308 by task systemd/1 psi_trigger_poll+0x3c/0x140 cgroup_pressure_poll+0x70/0xa0 cgroup_file_poll+0x8c/0x100 kernfs_fop_poll+0x11c/0x1c0 ep_item_poll.isra.0+0x188/0x2c0 Allocated by task 1: cgroup_file_open+0x88/0x388 kernfs_fop_open+0x73c/0xaf0 do_dentry_open+0x5fc/0x1200 vfs_open+0xa0/0x3f0 do_open+0x7e8/0xd08 path_openat+0x2fc/0x6b0 do_filp_open+0x174/0x368 Freed by task 8462: cgroup_file_release+0x130/0x1f8 kernfs_drain_open_files+0x17c/0x440 kernfs_drain+0x2dc/0x360 kernfs_show+0x1b8/0x288 cgroup_file_show+0x150/0x268 cgroup_pressure_write+0x1dc/0x340 cgroup_file_write+0x274/0x548 Reproduction Steps: 1. Open test/cpu.pressure and establish epoll monitoring 2. Disable monitoring: echo 0 > test/cgroup.pressure 3. Re-enable monitoring: echo 1 > test/cgroup.pressure The race condition occurs because: 1. When cgroup.pressure is disabled (echo 0 > cgroup.pressure), it: - Releases PSI triggers via cgroup_file_release() - Frees of->priv through kernfs_drain_open_files() 2. While epoll still holds reference to the file and continues polling 3. Re-enabling (echo 1 > cgroup.pressure) accesses freed of->priv epolling disable/enable cgroup.pressure fd=open(cpu.pressure) while(1) ... epoll_wait kernfs_fop_poll kernfs_get_active = true echo 0 > cgroup.pressure ... cgroup_file_show kernfs_show // inactive kn kernfs_drain_open_files cft->release(of); kfree(ctx); ... kernfs_get_active = false echo 1 > cgroup.pressure kernfs_show kernfs_activate_one(kn); kernfs_fop_poll kernfs_get_active = true cgroup_file_poll psi_trigger_poll // UAF ... end: close(fd) To address this issue, introduce kernfs_get_active_of() for kernfs open files to obtain active references. This function will fail if the open file has been released. Replace kernfs_get_active() with kernfs_get_active_of() to prevent further operations on released file descriptors.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 7018 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 34f26a15611afb03c33df6819359d36f5b382589 , < 34d9cafd469c69ad85e6a36b4303c78382cf5c79 (git)
Affected: 34f26a15611afb03c33df6819359d36f5b382589 , < 854baafc00c433cccbe0ab4231b77aeb9b637b77 (git)
Affected: 34f26a15611afb03c33df6819359d36f5b382589 , < 7e64474aba78d240f7804f48f2d454dcca78b15f (git)
Affected: 34f26a15611afb03c33df6819359d36f5b382589 , < ac5cda4fae8818cf1963317bb699f7f2f85b60af (git)
Affected: 34f26a15611afb03c33df6819359d36f5b382589 , < 3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f (git)
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Linux Linux Affected: 6.1
Unaffected: 0 , < 6.1 (semver)
Unaffected: 6.1.153 , ≤ 6.1.* (semver)
Unaffected: 6.6.107 , ≤ 6.6.* (semver)
Unaffected: 6.12.48 , ≤ 6.12.* (semver)
Unaffected: 6.16.8 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nkernfs: Fix UAF in polling when open file is released\n\nA use-after-free (UAF) vulnerability was identified in the PSI (Pressure\nStall Information) monitoring mechanism:\n\nBUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140\nRead of size 8 at addr ffff3de3d50bd308 by task systemd/1\n\npsi_trigger_poll+0x3c/0x140\ncgroup_pressure_poll+0x70/0xa0\ncgroup_file_poll+0x8c/0x100\nkernfs_fop_poll+0x11c/0x1c0\nep_item_poll.isra.0+0x188/0x2c0\n\nAllocated by task 1:\ncgroup_file_open+0x88/0x388\nkernfs_fop_open+0x73c/0xaf0\ndo_dentry_open+0x5fc/0x1200\nvfs_open+0xa0/0x3f0\ndo_open+0x7e8/0xd08\npath_openat+0x2fc/0x6b0\ndo_filp_open+0x174/0x368\n\nFreed by task 8462:\ncgroup_file_release+0x130/0x1f8\nkernfs_drain_open_files+0x17c/0x440\nkernfs_drain+0x2dc/0x360\nkernfs_show+0x1b8/0x288\ncgroup_file_show+0x150/0x268\ncgroup_pressure_write+0x1dc/0x340\ncgroup_file_write+0x274/0x548\n\nReproduction Steps:\n1. Open test/cpu.pressure and establish epoll monitoring\n2. Disable monitoring: echo 0 \u003e test/cgroup.pressure\n3. Re-enable monitoring: echo 1 \u003e test/cgroup.pressure\n\nThe race condition occurs because:\n1. When cgroup.pressure is disabled (echo 0 \u003e cgroup.pressure), it:\n   - Releases PSI triggers via cgroup_file_release()\n   - Frees of-\u003epriv through kernfs_drain_open_files()\n2. While epoll still holds reference to the file and continues polling\n3. Re-enabling (echo 1 \u003e cgroup.pressure) accesses freed of-\u003epriv\n\nepolling\t\t\tdisable/enable cgroup.pressure\nfd=open(cpu.pressure)\nwhile(1)\n...\nepoll_wait\nkernfs_fop_poll\nkernfs_get_active = true\techo 0 \u003e cgroup.pressure\n...\t\t\t\tcgroup_file_show\n\t\t\t\tkernfs_show\n\t\t\t\t// inactive kn\n\t\t\t\tkernfs_drain_open_files\n\t\t\t\tcft-\u003erelease(of);\n\t\t\t\tkfree(ctx);\n\t\t\t\t...\nkernfs_get_active = false\n\t\t\t\techo 1 \u003e cgroup.pressure\n\t\t\t\tkernfs_show\n\t\t\t\tkernfs_activate_one(kn);\nkernfs_fop_poll\nkernfs_get_active = true\ncgroup_file_poll\npsi_trigger_poll\n// UAF\n...\nend: close(fd)\n\nTo address this issue, introduce kernfs_get_active_of() for kernfs open\nfiles to obtain active references. This function will fail if the open file\nhas been released. Replace kernfs_get_active() with kernfs_get_active_of()\nto prevent further operations on released file descriptors."
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              "value": "AV:L - The attack requires local access to the cgroup2 filesystem \u2014 opening `cpu.pressure` with epoll and writing to `cgroup.pressure` via ordinary file syscalls. There is no network-facing path to `kernfs_fop_poll()`/`cgroup_pressure_write()`.\nAC:L - The reproduction is a deterministic three-step sequence (open+epoll, `echo 0 \u003e cgroup.pressure`, `echo 1 \u003e cgroup.pressure`) fully driven by the attacker, with no timing window to win. The gap between the `kfree()` and the re-activated poll is attacker-controlled and arbitrarily long, allowing reliable heap spraying of the kmalloc-192 slot.\nPR:L - An unprivileged local user suffices: `cgroup_add_file()` stamps created cgroup files with `current_fsuid()`, so any user with a delegated subtree (systemd `user@$UID.service`, containers, rootless podman) owns and can write both `cgroup.pressure` and `cpu.pressure` in a cgroup they `mkdir`. No capability check exists on this path, and cgroup2 is `FS_USERNS_MOUNT`.\nUI:N - The attacker performs the poll and both `cgroup.pressure` writes itself; no victim action is needed. (A privileged poller such as systemd/systemd-oomd can additionally be victimized, but that is not required.)\nS:U - The freed object, the corrupting write, and the resulting compromise all live in the kernel\u0027s own security authority. No VM, IOMMU, or sandbox boundary is crossed.\nC:H - The stale `of-\u003epriv` yields an attacker-controlled `struct psi_trigger *` in `psi_trigger_poll()`, and the subsequent `t-\u003eof` / `of_on(of)` / `atomic_read(\u0026on-\u003eevent)` dereferences turn that into an arbitrary kernel-memory read primitive, disclosing sensitive kernel data.\nI:H - `poll_wait()` on the forged `t-\u003eevent_wait` inserts a wait-queue entry at an attacker-chosen address (arbitrary linked-list write), `cmpxchg(\u0026t-\u003eevent, 1, 0)` adds a conditional write, and the eventual `wake_up()` calls a `wait_queue_entry-\u003efunc` pointer from corrupted memory \u2014 a viable control-flow hijack and local privilege escalation.\nA:H - Even without exploitation the use-after-free reliably produces a KASAN slab-use-after-free / oops on a wild pointer dereference, and a failed spray attempt panics the kernel \u2014 a full denial of service triggerable at will by an unprivileged user."
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          {
            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nkernfs: Fix UAF in polling when open file is released\n\nA use-after-free (UAF) vulnerability was identified in the PSI (Pressure\nStall Information) monitoring mechanism:\n\nBUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140\nRead of size 8 at addr ffff3de3d50bd308 by task systemd/1\n\npsi_trigger_poll+0x3c/0x140\ncgroup_pressure_poll+0x70/0xa0\ncgroup_file_poll+0x8c/0x100\nkernfs_fop_poll+0x11c/0x1c0\nep_item_poll.isra.0+0x188/0x2c0\n\nAllocated by task 1:\ncgroup_file_open+0x88/0x388\nkernfs_fop_open+0x73c/0xaf0\ndo_dentry_open+0x5fc/0x1200\nvfs_open+0xa0/0x3f0\ndo_open+0x7e8/0xd08\npath_openat+0x2fc/0x6b0\ndo_filp_open+0x174/0x368\n\nFreed by task 8462:\ncgroup_file_release+0x130/0x1f8\nkernfs_drain_open_files+0x17c/0x440\nkernfs_drain+0x2dc/0x360\nkernfs_show+0x1b8/0x288\ncgroup_file_show+0x150/0x268\ncgroup_pressure_write+0x1dc/0x340\ncgroup_file_write+0x274/0x548\n\nReproduction Steps:\n1. Open test/cpu.pressure and establish epoll monitoring\n2. Disable monitoring: echo 0 \u003e test/cgroup.pressure\n3. Re-enable monitoring: echo 1 \u003e test/cgroup.pressure\n\nThe race condition occurs because:\n1. When cgroup.pressure is disabled (echo 0 \u003e cgroup.pressure), it:\n   - Releases PSI triggers via cgroup_file_release()\n   - Frees of-\u003epriv through kernfs_drain_open_files()\n2. While epoll still holds reference to the file and continues polling\n3. Re-enabling (echo 1 \u003e cgroup.pressure) accesses freed of-\u003epriv\n\nepolling\t\t\tdisable/enable cgroup.pressure\nfd=open(cpu.pressure)\nwhile(1)\n...\nepoll_wait\nkernfs_fop_poll\nkernfs_get_active = true\techo 0 \u003e cgroup.pressure\n...\t\t\t\tcgroup_file_show\n\t\t\t\tkernfs_show\n\t\t\t\t// inactive kn\n\t\t\t\tkernfs_drain_open_files\n\t\t\t\tcft-\u003erelease(of);\n\t\t\t\tkfree(ctx);\n\t\t\t\t...\nkernfs_get_active = false\n\t\t\t\techo 1 \u003e cgroup.pressure\n\t\t\t\tkernfs_show\n\t\t\t\tkernfs_activate_one(kn);\nkernfs_fop_poll\nkernfs_get_active = true\ncgroup_file_poll\npsi_trigger_poll\n// UAF\n...\nend: close(fd)\n\nTo address this issue, introduce kernfs_get_active_of() for kernfs open\nfiles to obtain active references. This function will fail if the open file\nhas been released. Replace kernfs_get_active() with kernfs_get_active_of()\nto prevent further operations on released file descriptors."
          },
          {
            "lang": "es",
            "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nkernfs: Soluciona UAF en el sondeo cuando se libera un archivo abierto\n\nUna vulnerabilidad de uso despu\u00e9s de liberaci\u00f3n (UAF) fue identificada en el mecanismo de monitoreo PSI (Pressure Stall Information):\n\nBUG: KASAN: slab-use-after-free en psi_trigger_poll+0x3c/0x140\nLectura de tama\u00f1o 8 en la direcci\u00f3n ffff3de3d50bd308 por la tarea systemd/1\n\npsi_trigger_poll+0x3c/0x140\ncgroup_pressure_poll+0x70/0xa0\ncgroup_file_poll+0x8c/0x100\nkernfs_fop_poll+0x11c/0x1c0\nep_item_poll.isra.0+0x188/0x2c0\n\nAsignado por la tarea 1:\ncgroup_file_open+0x88/0x388\nkernfs_fop_open+0x73c/0xaf0\ndo_dentry_open+0x5fc/0x1200\nvfs_open+0xa0/0x3f0\ndo_open+0x7e8/0xd08\npath_openat+0x2fc/0x6b0\ndo_filp_open+0x174/0x368\n\nLiberado por la tarea 8462:\ncgroup_file_release+0x130/0x1f8\nkernfs_drain_open_files+0x17c/0x440\nkernfs_drain+0x2dc/0x360\nkernfs_show+0x1b8/0x288\ncgroup_file_show+0x150/0x268\ncgroup_pressure_write+0x1dc/0x340\ncgroup_file_write+0x274/0x548\n\nPasos de Reproducci\u00f3n:\n1. Abrir test/cpu.pressure y establecer monitoreo epoll\n2. Deshabilitar el monitoreo: echo 0 \u003e test/cgroup.pressure\n3. Volver a habilitar el monitoreo: echo 1 \u003e test/cgroup.pressure\n\nLa condici\u00f3n de carrera ocurre porque:\n1. Cuando cgroup.pressure est\u00e1 deshabilitado (echo 0 \u003e cgroup.pressure), este:\n   - Libera los disparadores PSI a trav\u00e9s de cgroup_file_release()\n   - Libera of-\u003epriv a trav\u00e9s de kernfs_drain_open_files()\n2. Mientras epoll a\u00fan mantiene una referencia al archivo y contin\u00faa sondeando\n3. Al volver a habilitar (echo 1 \u003e cgroup.pressure) se accede a of-\u003epriv liberado\n\nepolling\t\t\tdeshabilitar/habilitar cgroup.pressure\nfd=open(cpu.pressure)\nwhile(1)\n...\nepoll_wait\nkernfs_fop_poll\nkernfs_get_active = true\techo 0 \u003e cgroup.pressure\n...\t\t\t\tcgroup_file_show\n\t\t\t\tkernfs_show\n\t\t\t\t// kn inactivo\n\t\t\t\tkernfs_drain_open_files\n\t\t\t\tcft-\u003erelease(of);\n\t\t\t\tkfree(ctx);\n\t\t\t\t...\nkernfs_get_active = false\n\t\t\t\techo 1 \u003e cgroup.pressure\n\t\t\t\tkernfs_show\n\t\t\t\tkernfs_activate_one(kn);\nkernfs_fop_poll\nkernfs_get_active = true\ncgroup_file_poll\npsi_trigger_poll\n// UAF\n...\nend: close(fd)\n\nPara abordar este problema, se introduce kernfs_get_active_of() para que los archivos abiertos de kernfs obtengan referencias activas. Esta funci\u00f3n fallar\u00e1 si el archivo abierto ha sido liberado. Reemplazar kernfs_get_active() con kernfs_get_active_of() para evitar operaciones adicionales en descriptores de archivo liberados."
          }
        ],
        "id": "CVE-2025-39881",
        "lastModified": "2026-10-19T21:10:00.160",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.8,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "LOW",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 1.8,
              "impactScore": 5.9,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            },
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.8,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "LOW",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 1.8,
              "impactScore": 5.9,
              "source": "nvd@nist.gov",
              "type": "Primary"
            }
          ]
        },
        "published": "2025-09-23T06:15:47.793",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "tags": [
              "Patch"
            ],
            "url": "https://git.kernel.org/stable/c/34d9cafd469c69ad85e6a36b4303c78382cf5c79"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "tags": [
              "Patch"
            ],
            "url": "https://git.kernel.org/stable/c/3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "tags": [
              "Patch"
            ],
            "url": "https://git.kernel.org/stable/c/7e64474aba78d240f7804f48f2d454dcca78b15f"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "tags": [
              "Patch"
            ],
            "url": "https://git.kernel.org/stable/c/854baafc00c433cccbe0ab4231b77aeb9b637b77"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "tags": [
              "Patch"
            ],
            "url": "https://git.kernel.org/stable/c/ac5cda4fae8818cf1963317bb699f7f2f85b60af"
          },
          {
            "source": "af854a3a-2127-422b-91ae-364da2661108",
            "tags": [
              "Third Party Advisory"
            ],
            "url": "https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Modified",
        "weaknesses": [
          {
            "description": [
              {
                "lang": "en",
                "value": "CWE-416"
              }
            ],
            "source": "nvd@nist.gov",
            "type": "Primary"
          }
        ]
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-07-30T08:58:10+00:00",
      "cve": "CVE-2025-39881",
      "id": "CVE-2025-39881",
      "initial_release_date": "2025-09-23T00:00:00+00:00",
      "product_status:fixed": "1224",
      "product_status:known_not_affected": "112",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: kernfs: Fix UAF in polling when open file is released",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-39881.json",
      "version": "3"
    }
  }
}



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