{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21887",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-21887"
  ],
  "published": "2025-03-27T15:01:31Z",
  "summary": "ovl: fix UAF in ovl_dentry_update_reval by moving dput() in ovl_link_up",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\novl: fix UAF in ovl_dentry_update_reval by moving dput() in ovl_link_up\n\nThe issue was caused by dput(upper) being called before\novl_dentry_update_reval(), while upper->d_flags was still\naccessed in ovl_dentry_remote().\n\nMove dput(upper) after its last use to prevent use-after-free.\n\nBUG: KASAN: slab-use-after-free in ovl_dentry_remote fs/overlayfs/util.c:162 [inline]\nBUG: KASAN: slab-use-after-free in ovl_dentry_update_reval+0xd2/0xf0 fs/overlayfs/util.c:167\n\nCall Trace:\n <TASK>\n __dump_stack lib/dump_stack.c:88 [inline]\n dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:114\n print_address_description mm/kasan/report.c:377 [inline]\n print_report+0xc3/0x620 mm/kasan/report.c:488\n kasan_report+0xd9/0x110 mm/kasan/report.c:601\n ovl_dentry_remote fs/overlayfs/util.c:162 [inline]\n ovl_dentry_update_reval+0xd2/0xf0 fs/overlayfs/util.c:167\n ovl_link_up fs/overlayfs/copy_up.c:610 [inline]\n ovl_copy_up_one+0x2105/0x3490 fs/overlayfs/copy_up.c:1170\n ovl_copy_up_flags+0x18d/0x200 fs/overlayfs/copy_up.c:1223\n ovl_rename+0x39e/0x18c0 fs/overlayfs/dir.c:1136\n vfs_rename+0xf84/0x20a0 fs/namei.c:4893\n...\n </TASK>",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21887"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/f77618291836168eca99e89cd175256f928f5e64"
    },
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      "url": "https://git.kernel.org/stable/c/4b49d939b5a79117f939b77cc67efae2694d9799"
    },
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      "url": "https://git.kernel.org/stable/c/a7c41830ffcd17b2177a95a9b99b270302090c35"
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      "url": "https://git.kernel.org/stable/c/64455c8051c3aedc71abb7ec8d47c80301f99f00"
    },
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      "url": "https://git.kernel.org/stable/c/3594aad97e7be2557ca9fa9c931b206b604028c8"
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    },
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    "upstream_modified": "2026-08-05T12:19:11Z",
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      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The bug is reached through local syscalls only \u2014 mounting overlayfs (and a FUSE upper) and issuing rename(2)/open(O_TRUNC) to trigger copy-up. No network-facing component processes attacker data on this path.\nAC:L - The attacker controls both halves of the timing: the FUSE server sets entry_valid=0 so dput() deterministically kills the dentry, and it stalls the SETATTR in ovl_set_timestamps() to hold the window open arbitrarily long for RCU to free the object and for a heap spray to refill the slot. No condition depends on chance or on unrelated system state.\nPR:L - Both overlayfs and fuse are FS_USERNS_MOUNT, so an ordinary unprivileged user can create the whole setup inside `unshare -Urm`, and several distros permit unprivileged overlayfs outright; the copy-up path contains no capability check. Some form of local account is required, so not None.\nUI:N - The attacker performs the mount, the FUSE service, and the rename entirely on their own, within their own namespace. No victim action of any kind is needed.\nS:U - The corrupted and disclosed state is kernel memory reached from a kernel-mode fault; the impact stays within the kernel's own security authority with no crossing into a hypervisor, IOMMU, or other authority.\nC:H - This is a slab use-after-free read of a freed struct dentry whose contents the attacker can shape by spraying the dentry_cache during the FUSE-controlled stall, and the read value is propagated into live VFS state, yielding an observable oracle on freed kernel memory. Per UAF scoring guidance this is High.\nI:H - Content read out of a freed, attacker-refillable slab object is written into a live dentry's d_flags, corrupting VFS revalidation state (stale dentries accepted on remote upper/lower filesystems). Use-after-free primitives of this class are treated as High integrity impact.\nA:H - KASAN confirms a slab-use-after-free; once the dentry slab page is returned to the page allocator \u2014 which the attacker can encourage by draining the dcache during the stall \u2014 the dereference faults and oopses the kernel, and the fault occurs with the upper directory inode lock held, wedging further operations."
    }
  }
}