# CVE-2021-47303: bpf: Track subprog poke descriptors correctly and fix use-after-free

> A use-after-free vulnerability in the Linux kernel BPF subsystem allows attackers to execute arbitrary code or escalate privileges.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 21 May 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47303, GHSA-w825-9xqr-f6q3, UBUNTU-CVE-2021-47303

## What is CVE-2021-47303?

CVE-2021-47303 is a high-severity use after free vulnerability in Linux kernel, published on 21 May 2024. 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:

bpf: Track subprog poke descriptors correctly and fix use-after-free

Subprograms are calling map_poke_track(), but on program release there is no
hook to call map_poke_untrack(). However, on program release, the aux memory
(and poke descriptor table) is freed even though we still have a reference to
it in the element list of the map aux data. When we run map_poke_run(), we then
end up accessing free'd memory, triggering KASAN in prog_array_map_poke_run():

  [...]
  [  402.824689] BUG: KASAN: use-after-free in prog_array_map_poke_run+0xc2/0x34e
  [  402.824698] Read of size 4 at addr ffff8881905a7940 by task hubble-fgs/4337
  [  402.824705] CPU: 1 PID: 4337 Comm: hubble-fgs Tainted: G          I       5.12.0+ #399
  [  402.824715] Call Trace:
  [  402.824719]  dump_stack+0x93/0xc2
  [  402.824727]  print_address_description.constprop.0+0x1a/0x140
  [  402.824736]  ? prog_array_map_poke_run+0xc2/0x34e
  [  402.824740]  ? prog_array_map_poke_run+0xc2/0x34e
  [  402.824744]  kasan_report.cold+0x7c/0xd8
  [  402.824752]  ? prog_array_map_poke_run+0xc2/0x34e
  [  402.824757]  prog_array_map_poke_run+0xc2/0x34e
  [  402.824765]  bpf_fd_array_map_update_elem+0x124/0x1a0
  [...]

The elements concerned are walked as follows:

    for (i = 0; i < elem->aux->size_poke_tab; i++) {
           poke = &elem->aux->poke_tab[i];
    [...]

The access to size_poke_tab is a 4 byte read, verified by checking offsets
in the KASAN dump:

  [  402.825004] The buggy address belongs to the object at ffff8881905a7800
                 which belongs to the cache kmalloc-1k of size 1024
  [  402.825008] The buggy address is located 320 bytes inside of
                 1024-byte region [ffff8881905a7800, ffff8881905a7c00)

The pahole output of bpf_prog_aux:

  struct bpf_prog_aux {
    [...]
    /* --- cacheline 5 boundary (320 bytes) --- */
    u32                        size_poke_tab;        /*   320     4 */
    [...]

In general, subprograms do not necessarily manage their own data structures.
For example, BTF func_info and linfo are just pointers to the main program
structure. This allows reference counting and cleanup to be done on the latter
which simplifies their management a bit. The aux->poke_tab struct, however,
did not follow this logic. The initial proposed fix for this use-after-free
bug further embedded poke data tracking into the subprogram with proper
reference counting. However, Daniel and Alexei questioned why we were treating
these objects special; I agree, its unnecessary. The fix here removes the per
subprogram poke table allocation and map tracking and instead simply points
the aux->poke_tab pointer at the main programs poke table. This way, map
tracking is simplified to the main program and we do not need to manage them
per subprogram.

This also means, bpf_prog_free_deferred(), which unwinds the program reference
counting and kfrees objects, needs to ensure that we don't try to double free
the poke_tab when free'ing the subprog structures. This is easily solved by
NULL'ing the poke_tab pointer. The second detail is to ensure that per
subprogram JIT logic only does fixups on poke_tab[] entries it owns. To do
this, we add a pointer in the poke structure to point at the subprogram value
so JITs can easily check while walking the poke_tab structure if the current
entry belongs to the current program. The aux pointer is stable and therefore
suitable for such comparison. On the jit_subprogs() error path, we omit
cleaning up the poke->aux field because these are only ever referenced from
the JIT side, but on error we will never make it to the JIT, so its fine to
leave them dangling. Removing these pointers would complicate the error path
for no reason. However, we do need to untrack all poke descriptors from the
main program as otherwise they could race with the freeing of JIT memory from
the subprograms. Lastly, a748c6975dea3 ("bpf: propagate poke des
---truncated---

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-47303?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47303?

Apply the vendor's patch.

## Is CVE-2021-47303 being exploited?

There is no known exploitation of CVE-2021-47303 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47303 and fix it if it is affected.

What it is: CVE-2021-47303 is a high-severity use after free vulnerability in Linux kernel, published on 21 May 2024. 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-2021-47303 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2021-47303/ (Markdown: https://insights.sbomify.com/CVE-2021-47303/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47303
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47303
- OSV: https://osv.dev/vulnerability/CVE-2021-47303

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47303/
Summaries are generated and may contain mistakes; the official description above is the record.