GHSA-GW8W-5RWC-5FFX

Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-13 09:32
VLAI
Details

In the Linux kernel, the following vulnerability has been resolved:

nfsd: fix UAF in async copy cancel and shutdown

An async copy could be freed or used after free while a teardown caller (OFFLOAD_CANCEL, nfsd4_shutdown_copy, nfsd4_cancel_copy_by_sb) raced the copy kthread:

  • find_async_copy() bumped copy->refcount but left the copy on clp->async_copies, so the reaper's cleanup_async_copy() could run release_copy_files() concurrently with a cancel/shutdown caller. Both put and NULL nf_src/nf_dst without a common lock, double-putting the nfsd_file and freeing it early.

  • nfsd4_do_async_copy() set NFSD4_COPY_F_STOPPED before its final uses of the copy (nfsd_update_cmtime_attr() on copy->nf_dst, nfsd4_send_cb_offload()). nfsd4_stop_copy() treats a set STOPPED bit as "kthread done, skip kthread_stop()", so a teardown caller ran release_copy_files() -- which puts and NULLs nf_dst -- while the kthread still dereferenced it (NULL/UAF).

  • copy->copy_task was never pinned. The one-shot kthread self-reaps on return, so kthread_stop()'s get_task_struct() could touch a freed task_struct.

  • co_cb is embedded in the copy, but nfsd4_send_cb_offload() held a reference only on the client, so a concurrent teardown could free the copy while the CB_OFFLOAD callback was in flight.

Fix the teardown lifetime as a whole:

  • find_async_copy() unlinks the copy (clear cp_clp, list_del_init) under async_lock; the cancel, shutdown, and sb-cancel paths drop the list-membership reference via nfs4_put_copy() after nfsd4_stop_copy(). Drop the now-redundant list_del fixup from cleanup_async_copy().

  • Because unlinking hides the copy from the reaper, its cleanup_async_copy() can no longer remove the copy's s2s_cp_stateids entry; the cancel/shutdown/sb-cancel paths now call nfs4_free_copy_state() themselves (while cp_clp is still valid) so the entry does not dangle at freed memory for the laundromat and manage_cpntf_state() to dereference.

  • Give the kthread its own reference, taken in nfsd4_copy() before wake_up_process() and dropped at the end of nfsd4_do_async_copy(); call wake_up_process() before list_add().

  • Pin the task_struct with get_task_struct() in nfsd4_copy(), released in nfs4_put_copy(), so kthread_stop() is safe whenever the kthread exits. Set NFSD4_COPY_F_STOPPED only in nfsd4_stop_copy(), which now always kthread_stop()s before release_copy_files(); completion is still reported via NFSD4_COPY_F_COMPLETED, so nfsd4_has_active_async_copies() is unaffected. Each teardown caller removes the copy from clp->async_copies first, so kthread_stop() runs exactly once.

  • Take a copy reference in nfsd4_send_cb_offload(), dropped in nfsd4_cb_offload_release(). The kthread still holds its own reference there, so the refcount_inc() cannot race the final free.

  • Read cp_clp with smp_load_acquire() to pair with the unordered set_bit()/clear_bit() writers (Documentation/atomic_bitops.rst).

Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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{
  "affected": [],
  "aliases": [
    "CVE-2026-89675"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-09-11T20:19:54Z",
    "severity": "CRITICAL"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: fix UAF in async copy cancel and shutdown\n\nAn async copy could be freed or used after free while a teardown caller\n(OFFLOAD_CANCEL, nfsd4_shutdown_copy, nfsd4_cancel_copy_by_sb) raced the\ncopy kthread:\n\n  - find_async_copy() bumped copy-\u003erefcount but left the copy on\n    clp-\u003easync_copies, so the reaper\u0027s cleanup_async_copy() could run\n    release_copy_files() concurrently with a cancel/shutdown caller. Both\n    put and NULL nf_src/nf_dst without a common lock, double-putting the\n    nfsd_file and freeing it early.\n\n  - nfsd4_do_async_copy() set NFSD4_COPY_F_STOPPED before its final uses\n    of the copy (nfsd_update_cmtime_attr() on copy-\u003enf_dst,\n    nfsd4_send_cb_offload()). nfsd4_stop_copy() treats a set STOPPED bit\n    as \"kthread done, skip kthread_stop()\", so a teardown caller ran\n    release_copy_files() -- which puts and NULLs nf_dst -- while the\n    kthread still dereferenced it (NULL/UAF).\n\n  - copy-\u003ecopy_task was never pinned. The one-shot kthread self-reaps on\n    return, so kthread_stop()\u0027s get_task_struct() could touch a freed\n    task_struct.\n\n  - co_cb is embedded in the copy, but nfsd4_send_cb_offload() held a\n    reference only on the client, so a concurrent teardown could free\n    the copy while the CB_OFFLOAD callback was in flight.\n\nFix the teardown lifetime as a whole:\n\n  - find_async_copy() unlinks the copy (clear cp_clp, list_del_init)\n    under async_lock; the cancel, shutdown, and sb-cancel paths drop the\n    list-membership reference via nfs4_put_copy() after nfsd4_stop_copy().\n    Drop the now-redundant list_del fixup from cleanup_async_copy().\n\n  - Because unlinking hides the copy from the reaper, its\n    cleanup_async_copy() can no longer remove the copy\u0027s s2s_cp_stateids\n    entry; the cancel/shutdown/sb-cancel paths now call\n    nfs4_free_copy_state() themselves (while cp_clp is still valid) so\n    the entry does not dangle at freed memory for the laundromat and\n    manage_cpntf_state() to dereference.\n\n  - Give the kthread its own reference, taken in nfsd4_copy() before\n    wake_up_process() and dropped at the end of nfsd4_do_async_copy();\n    call wake_up_process() before list_add().\n\n  - Pin the task_struct with get_task_struct() in nfsd4_copy(), released\n    in nfs4_put_copy(), so kthread_stop() is safe whenever the kthread\n    exits. Set NFSD4_COPY_F_STOPPED only in nfsd4_stop_copy(), which now\n    always kthread_stop()s before release_copy_files(); completion is\n    still reported via NFSD4_COPY_F_COMPLETED, so\n    nfsd4_has_active_async_copies() is unaffected. Each teardown caller\n    removes the copy from clp-\u003easync_copies first, so kthread_stop() runs\n    exactly once.\n\n  - Take a copy reference in nfsd4_send_cb_offload(), dropped in\n    nfsd4_cb_offload_release(). The kthread still holds its own reference\n    there, so the refcount_inc() cannot race the final free.\n\n  - Read cp_clp with smp_load_acquire() to pair with the unordered\n    set_bit()/clear_bit() writers (Documentation/atomic_bitops.rst).",
  "id": "GHSA-gw8w-5rwc-5ffx",
  "modified": "2026-09-13T09:32:27Z",
  "published": "2026-09-11T21:31:36Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89675"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/62c0f6eaf050bb9284c1f9cac6ed1770092e6b95"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/9031493ef7369d5c59c4bacc96f0c85965f09a98"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a385cf5e016b748babf94cc664e43a26e17db117"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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