drm/drm_file: Fix pid refcounting race

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  "id": "c00ea1287074260dd570556bfa7bbf660fe01e11fdb8fedd1696c1666fdb80d0",
  "url": "https://osv.dev/vulnerability/CVE-2024-39486",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "drm/drm_file: Fix pid refcounting race",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: drm/drm_file: Fix pid refcounting race , Maxime Ripard , Thomas Zimmermann filp->pid is supposed to be a refcounted pointer; however, before this patch, drm_file_update_pid() only increments the refcount of a struct pid after storing a pointer to it in filp->pid and dropping the dev->filelist_mutex, making the following race possible: process A process B ========= ========= begin drm_file_update_pid mutex_lock(&dev->filelist_mutex) rcu_replace_pointer(filp->pid, , 1) mutex_unlock(&dev->filelist_mutex) begin drm_file_update_pid mutex_lock(&dev->filelist_mutex) rcu_replace_pointer(filp->pid, , 1) mutex_unlock(&dev->filelist_mutex) get_pid() synchronize_rcu() put_pid() *** pid B reaches refcount 0 and is freed here *** get_pid() *** UAF *** synchronize_rcu() put_pid() As far as I know, this race can only occur with CONFIG_PREEMPT_RCU=y because it requires RCU to detect a quiescent state in code that is not explicitly calling into the scheduler. This race leads to use-after-free of a \"struct pid\". It is probably somewhat hard to hit because process A has to pass through a synchronize_rcu() operation while process B is between mutex_unlock() and get_pid(). Fix it by ensuring that by the time a pointer to the current task's pid is stored in the file, an extra reference to the pid has been taken. This fix also removes the condition for synchronize_rcu(); I think that optimization is unnecessary complexity, since in ",
    "evidence": "osv:CVE-2024-39486",
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      "modified": "2026-10-28T02:30:51.399473029Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/0acce2a5c619ef1abdee783d7fea5eac78ce4844",
        "https://git.kernel.org/stable/c/16682588ead4a593cf1aebb33b36df4d1e9e4ffa",
        "https://git.kernel.org/stable/c/4f2a129b33a2054e62273edd5a051c34c08d96e9",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2024/39xxx/CVE-2024-39486.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2024-39486",
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  "source_id": "core-osv",
  "document_id": 519633,
  "published_at": "2024-07-06T09:25:21.514+00:00"
}