CVE-2025-39941 (GCVE-0-2025-39941)

Vulnerability from cvelistv5 – Published: 2025-10-04 07:31 – Updated: 2026-08-05 12:06
VLAI
Title
zram: fix slot write race condition
Summary
In the Linux kernel, the following vulnerability has been resolved: zram: fix slot write race condition Parallel concurrent writes to the same zram index result in leaked zsmalloc handles. Schematically we can have something like this: CPU0 CPU1 zram_slot_lock() zs_free(handle) zram_slot_lock() zram_slot_lock() zs_free(handle) zram_slot_lock() compress compress handle = zs_malloc() handle = zs_malloc() zram_slot_lock zram_set_handle(handle) zram_slot_lock zram_slot_lock zram_set_handle(handle) zram_slot_lock Either CPU0 or CPU1 zsmalloc handle will leak because zs_free() is done too early. In fact, we need to reset zram entry right before we set its new handle, all under the same slot lock scope.
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 6639 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 71268035f5d734ad6373d953298bd5779985497a , < ff750e9f2c4d63854c33967d1646b5e89a9a19a2 (git)
Affected: 71268035f5d734ad6373d953298bd5779985497a , < ce4be9e4307c5a60701ff6e0cafa74caffdc54ce (git)
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Linux Linux Affected: 6.14
Unaffected: 0 , < 6.14 (semver)
Unaffected: 6.16.9 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
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Show details on NVD website
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              "value": "AV:L - The vulnerable code is the zram block-device write path (`zram_submit_bio` \u2192 `zram_write_page`), reached only through local block I/O \u2014 writes to /dev/zramN, a zram-backed filesystem, or zram swap. There is no network-facing consumer of this path.\nAC:L - The attacker controls both sides of the race: two threads issuing overlapping O_DIRECT or sub-page writes to the same 4 KiB index in a loop, with the window widened by the compress/zs_malloc stage and further by `zram_bvec_write_partial()`\u0027s read-modify-write. The reporter reproduced it with an ordinary fio/blktests workload, and no attacker-uncontrollable condition is involved.\nPR:L - No capability check exists on the write path; the attacker only needs write access to a zram-backed device or filesystem, which unprivileged users routinely have on the dominant zram deployments (Android/ChromeOS swap, systemd zram-generator, embedded/automotive zram-backed /tmp, /var, or overlay upper dirs), or via membership in the `disk` group. Real root in the init namespace is not required.\nUI:N - The attacker\u0027s own two threads create and win the race entirely on their own. No victim action, mount, or file-open by another user is needed.\nS:U - The corruption is confined to the zram driver\u0027s slot table and zsmalloc pool within the kernel\u0027s own security authority. No VM, IOMMU, or sandbox boundary is crossed.\nC:H - The flag/handle desync yields two distinct kernel-memory disclosures into userspace-readable pages: a sticky ZRAM_SAME slot holding a real handle fills a whole 4 KiB page with a raw kernel heap pointer, and a sticky ZRAM_HUGE slot holding a small compressed object makes `read_incompressible_page()` `copy_page()` up to ~4 KiB of adjacent kernel memory past the end of the zspage.\nI:H - The race corrupts kernel slot metadata (handle, ZRAM_SAME/ZRAM_HUGE flags, obj_size) and the pool accounting counters, so stored page contents are silently replaced by kernel pointers or out-of-bounds heap data \u2014 corrupting file blocks or swapped-out anonymous pages belonging to other, potentially privileged, processes.\nA:H - `zs_obj_read_begin(pool, handle, NULL)` on a desynced ZRAM_HUGE slot whose object spans two zpdescs memcpys into a NULL `local_copy`, producing a kernel oops, alongside `WARN_ON` splats on decompression failure. Additionally, every won race permanently orphans a zsmalloc handle, an unbounded and repeatable leak that exhausts memory on the very device backing system swap."
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            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nzram: fix slot write race condition\n\nParallel concurrent writes to the same zram index result in leaked\nzsmalloc handles.  Schematically we can have something like this:\n\nCPU0                              CPU1\nzram_slot_lock()\nzs_free(handle)\nzram_slot_lock()\n\t\t\t\tzram_slot_lock()\n\t\t\t\tzs_free(handle)\n\t\t\t\tzram_slot_lock()\n\ncompress\t\t\tcompress\nhandle = zs_malloc()\t\thandle = zs_malloc()\nzram_slot_lock\nzram_set_handle(handle)\nzram_slot_lock\n\t\t\t\tzram_slot_lock\n\t\t\t\tzram_set_handle(handle)\n\t\t\t\tzram_slot_lock\n\nEither CPU0 or CPU1 zsmalloc handle will leak because zs_free() is done\ntoo early.  In fact, we need to reset zram entry right before we set its\nnew handle, all under the same slot lock scope."
          },
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            "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nzram: corregir condici\u00f3n de carrera de escritura de slot\n\nLas escrituras concurrentes paralelas al mismo \u00edndice de zram resultan en handles de zsmalloc filtrados. Esquem\u00e1ticamente podemos tener algo como esto:\n\nCPU0                              CPU1\nzram_slot_lock()\nzs_free(handle)\nzram_slot_lock()\n\t\t\t\tzram_slot_lock()\n\t\t\t\tzs_free(handle)\n\t\t\t\tzram_slot_lock()\n\ncompress\t\t\tcompress\nhandle = zs_malloc()\t\thandle = zs_malloc()\nzram_slot_lock\nzram_set_handle(handle)\nzram_slot_lock\n\t\t\t\tzram_slot_lock\n\t\t\t\tzram_set_handle(handle)\n\t\t\t\tzram_slot_lock\n\nEl handle de zsmalloc de la CPU0 o la CPU1 se filtrar\u00e1 porque zs_free() se ejecuta demasiado pronto. De hecho, necesitamos restablecer la entrada de zram justo antes de establecer su nuevo handle, todo bajo el mismo \u00e1mbito de bloqueo de slot."
          }
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        "id": "CVE-2025-39941",
        "lastModified": "2026-10-19T21:10:00.160",
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      "aggregate_severity": "Low",
      "current_release_date": "2026-11-07T13:25:10+00:00",
      "cve": "CVE-2025-39941",
      "id": "CVE-2025-39941",
      "initial_release_date": "2025-10-04T00:00:00+00:00",
      "product_status:known_not_affected": "200",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: zram: fix slot write race condition",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-39941.json",
      "version": "3"
    }
  }
}



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