# CVE-2025-39984: net: tun: Update napi->skb after XDP process

> A use-after-free vulnerability in the Linux kernel's TUN driver allows local attackers to execute arbitrary code or escalate privileges.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Oct 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-39984, DEBIAN-CVE-2025-39984, GHSA-3cpf-qphm-hqhv, ROOT-OS-DEBIAN-13-CVE-2025-39984, UBUNTU-CVE-2025-39984

## What is CVE-2025-39984?

CVE-2025-39984 is an use after free vulnerability in Linux, published on 15 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

net: tun: Update napi->skb after XDP process

The syzbot report a UAF issue:

  BUG: KASAN: slab-use-after-free in skb_reset_mac_header include/linux/skbuff.h:3150 [inline]
  BUG: KASAN: slab-use-after-free in napi_frags_skb net/core/gro.c:723 [inline]
  BUG: KASAN: slab-use-after-free in napi_gro_frags+0x6e/0x1030 net/core/gro.c:758
  Read of size 8 at addr ffff88802ef22c18 by task syz.0.17/6079
  CPU: 0 UID: 0 PID: 6079 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
  Call Trace:
   <TASK>
   dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
   print_address_description mm/kasan/report.c:378 [inline]
   print_report+0xca/0x240 mm/kasan/report.c:482
   kasan_report+0x118/0x150 mm/kasan/report.c:595
   skb_reset_mac_header include/linux/skbuff.h:3150 [inline]
   napi_frags_skb net/core/gro.c:723 [inline]
   napi_gro_frags+0x6e/0x1030 net/core/gro.c:758
   tun_get_user+0x28cb/0x3e20 drivers/net/tun.c:1920
   tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
   new_sync_write fs/read_write.c:593 [inline]
   vfs_write+0x5c9/0xb30 fs/read_write.c:686
   ksys_write+0x145/0x250 fs/read_write.c:738
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

  Allocated by task 6079:
   kasan_save_stack mm/kasan/common.c:47 [inline]
   kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
   unpoison_slab_object mm/kasan/common.c:330 [inline]
   __kasan_mempool_unpoison_object+0xa0/0x170 mm/kasan/common.c:558
   kasan_mempool_unpoison_object include/linux/kasan.h:388 [inline]
   napi_skb_cache_get+0x37b/0x6d0 net/core/skbuff.c:295
   __alloc_skb+0x11e/0x2d0 net/core/skbuff.c:657
   napi_alloc_skb+0x84/0x7d0 net/core/skbuff.c:811
   napi_get_frags+0x69/0x140 net/core/gro.c:673
   tun_napi_alloc_frags drivers/net/tun.c:1404 [inline]
   tun_get_user+0x77c/0x3e20 drivers/net/tun.c:1784
   tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
   new_sync_write fs/read_write.c:593 [inline]
   vfs_write+0x5c9/0xb30 fs/read_write.c:686
   ksys_write+0x145/0x250 fs/read_write.c:738
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

  Freed by task 6079:
   kasan_save_stack mm/kasan/common.c:47 [inline]
   kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
   kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
   poison_slab_object mm/kasan/common.c:243 [inline]
   __kasan_slab_free+0x5b/0x80 mm/kasan/common.c:275
   kasan_slab_free include/linux/kasan.h:233 [inline]
   slab_free_hook mm/slub.c:2422 [inline]
   slab_free mm/slub.c:4695 [inline]
   kmem_cache_free+0x18f/0x400 mm/slub.c:4797
   skb_pp_cow_data+0xdd8/0x13e0 net/core/skbuff.c:969
   netif_skb_check_for_xdp net/core/dev.c:5390 [inline]
   netif_receive_generic_xdp net/core/dev.c:5431 [inline]
   do_xdp_generic+0x699/0x11a0 net/core/dev.c:5499
   tun_get_user+0x2523/0x3e20 drivers/net/tun.c:1872
   tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1996
   new_sync_write fs/read_write.c:593 [inline]
   vfs_write+0x5c9/0xb30 fs/read_write.c:686
   ksys_write+0x145/0x250 fs/read_write.c:738
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

After commit e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in
generic mode"), the original skb may be freed in skb_pp_cow_data() when
XDP program was attached, which was allocated in tun_napi_alloc_frags().
However, the napi->skb still point to the original skb, update it after
XDP process.

## Which versions are affected by CVE-2025-39984?


## How do I fix CVE-2025-39984?

Apply the vendor's patch.

## Is CVE-2025-39984 being exploited?

There is no known exploitation of CVE-2025-39984 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39984 and fix it if it is affected.

What it is: CVE-2025-39984 is an use after free vulnerability in Linux, published on 15 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-39984 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-39984/ (Markdown: https://insights.sbomify.com/CVE-2025-39984/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39984
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39984
- OSV: https://osv.dev/vulnerability/CVE-2025-39984

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39984/
Summaries are generated and may contain mistakes; the official description above is the record.